tools: Set the tool's default binary name

This patch: fafd3ec9c assumes that tools must build from
the main makefile folder.
This assumption leads to the error when somebody wants to
build a tool from the tool's folder.
Hence changes are done to provide the default binary name
in the tool's makefile.

Change-Id: Iae570a7f8d322151376b6feb19e739300eecc3fc
Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
diff --git a/tools/sptool/Makefile b/tools/sptool/Makefile
index f724c26..1fa85fb 100644
--- a/tools/sptool/Makefile
+++ b/tools/sptool/Makefile
@@ -8,6 +8,7 @@
 include ${MAKE_HELPERS_DIRECTORY}build_macros.mk
 include ${MAKE_HELPERS_DIRECTORY}build_env.mk
 
+SPTOOL  ?= sptool${BIN_EXT}
 PROJECT := $(notdir ${SPTOOL})
 OBJECTS := sptool.o
 V ?= 0