Make:Use environment variables for OS detection.

Add make helper files to select the appropriate settings for the build
environment. Selection is made in make_helpers/build_env.mk, which
selects other files to include using generic build environment settings.
The Trusted Firmware Makefile and supporting tool Makefiles are updated
to include build_env.mk instead of unix.mk.

NOTE: This change does not fully enable builds in other build
      environments. It facilitates this without compromising the
      existing build environments.

Change-Id: Ic4064ffe6ce158bbd16d7cc9f27dd4655a3580f6
diff --git a/make_helpers/windows.mk b/make_helpers/windows.mk
new file mode 100644
index 0000000..92cc8f6
--- /dev/null
+++ b/make_helpers/windows.mk
@@ -0,0 +1,93 @@
+#
+# Copyright (c) 2016, ARM Limited and Contributors. All rights reserved.
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions are met:
+#
+# Redistributions of source code must retain the above copyright notice, this
+# list of conditions and the following disclaimer.
+#
+# Redistributions in binary form must reproduce the above copyright notice,
+# this list of conditions and the following disclaimer in the documentation
+# and/or other materials provided with the distribution.
+#
+# Neither the name of ARM nor the names of its contributors may be used
+# to endorse or promote products derived from this software without specific
+# prior written permission.
+#
+# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+# AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+# IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+# ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
+# LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+# CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+# SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+# INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+# CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+# ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+# POSSIBILITY OF SUCH DAMAGE.
+#
+#
+
+# OS specific parts for builds in a Windows_NT environment. The
+# environment variable OS is set to Windows_NT on all modern Windows platforms
+
+# Include generic windows command definitions.
+
+ifndef WINDOWS_MK
+    WINDOWS_MK := $(lastword $(MAKEFILE_LIST))
+
+    ECHO_BLANK_LINE := @cmd /c echo.
+
+    DIR_DELIM := $(strip \)
+    PATH_SEP := ;
+
+    # For some Windows native commands there is a problem with the directory delimiter.
+    # Make uses / (slash) and the commands expect \ (backslash)
+    # We have to provide a means of translating these, so we define local functions.
+
+    # ${1} is the file to be copied.
+    # ${2} is the destination file name.
+    define SHELL_COPY
+	$(eval tmp_from_file:=$(subst /,\,${1}))
+	$(eval tmp_to_file:=$(subst /,\,${2}))
+	copy "${tmp_from_file}" "${tmp_to_file}"
+    endef
+
+    # ${1} is the directory to be copied.
+    # ${2} is the destination directory path.
+    define SHELL_COPY_TREE
+	$(eval tmp_from_dir:=$(subst /,\,${1}))
+	$(eval tmp_to_dir:=$(subst /,\,${2}))
+	xcopy /HIVE "${tmp_from_dir}" "${tmp_to_dir}"
+    endef
+
+    # ${1} is the file to be deleted.
+    define SHELL_DELETE
+	$(eval tmp_del_file:=$(subst /,\,${*}))
+	-@if exist $(tmp_del_file)  del /Q $(tmp_del_file)
+    endef
+
+    # ${1} is a space delimited list of files to be deleted.
+    define SHELL_DELETE_ALL
+	$(eval $(foreach filename,$(wildcard ${1}),$(call DELETE_IF_THERE,${filename})))
+    endef
+
+    # ${1} is the directory to be generated.
+    # ${2} is optional, and allows prerequisites to be specified.
+    define MAKE_PREREQ_DIR
+
+${1} : ${2}
+	$(eval tmp_dir:=$(subst /,\,${1}))
+	-@if not exist "$(tmp_dir)"  mkdir "${tmp_dir}"
+
+    endef
+
+    # ${1} is the directory to be removed.
+    define SHELL_REMOVE_DIR
+	$(eval tmp_dir:=$(subst /,\,${1}))
+	-@if exist "$(tmp_dir)"  rd /Q /S "$(tmp_dir)"
+    endef
+
+endif
+