refactor(build): update build string to show rc-tag

The build string has been updated to reflect the rc-tagged version.
The change has been made to be in line with the TF-A.
The build string has been changed now as follows:
	NOTICE:  2.6(fvp,debug):v2.7-rc0
and, previously it was:
	NOTICE:  v2.6(fvp,debug):207353c

Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
Change-Id: I79f31750f3bc6bacc118e9c567b5c9f9dd7bbf3f
diff --git a/Makefile b/Makefile
index d22e32e..16b13d0 100644
--- a/Makefile
+++ b/Makefile
@@ -67,7 +67,7 @@
 
 # Default build string (git branch and commit)
 ifeq (${BUILD_STRING},)
-        BUILD_STRING    :=      $(shell git log -n 1 --pretty=format:"%h")
+        BUILD_STRING  :=  $(shell git describe --always --dirty --tags 2> /dev/null)
 endif
 
 VERSION_STRING		:= 	v${VERSION_MAJOR}.${VERSION_MINOR}(${PLAT},${BUILD_TYPE}):${BUILD_STRING}