Use abspath to dereference $BUILD_BASE

If the user tries to change BUILD_BASE to put the build products outside
the build tree the compile will fail due to hard coded assumptions that
$BUILD_BASE is a relative path. Fix by using $(abspath $(BUILD_BASE))
to rationalize to an absolute path every time and remove the relative
path assumptions.

This patch also adds documentation that BUILD_BASE can be specified by
the user.

Signed-off-by: Grant Likely <grant.likely@arm.com>
Signed-off-by: Manish Pandey <manish.pandey2@arm.com>
Change-Id: Ib1af874de658484aaffc672f30029b852d2489c8
diff --git a/docs/getting_started/build-options.rst b/docs/getting_started/build-options.rst
index 630d861..153dc39 100644
--- a/docs/getting_started/build-options.rst
+++ b/docs/getting_started/build-options.rst
@@ -119,6 +119,8 @@
 -  ``BUILD_STRING``: Input string for VERSION_STRING, which allows the TF-A
    build to be uniquely identified. Defaults to the current git commit id.
 
+-  ``BUILD_BASE``: Output directory for the build. Defaults to ``./build``
+
 -  ``CFLAGS``: Extra user options appended on the compiler's command line in
    addition to the options set by the build system.