kbuild: rename SRCTREE to srctree

Prior to Kbuild, $(TOPDIR) or $(SRCTREE) was used for
pointing to the top of source directory.
(No difference between the two.)

In Kbuild style, $(srctree) is used for instead.
This commit renames SRCTREE to srctree and deletes the
defition of SRCTREE.

Note that SRCTREE in scripts/kernel-doc, scripts/docproc.c,
doc/DocBook/Makefile should be keep.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/tools/env/Makefile b/tools/env/Makefile
index 6ad81fd..fcb752d 100644
--- a/tools/env/Makefile
+++ b/tools/env/Makefile
@@ -12,7 +12,7 @@
 
 # Compile for a hosted environment on the target
 HOST_EXTRACFLAGS  = $(patsubst -I%,-idirafter%, $(UBOOTINCLUDE)) \
-		-idirafter $(SRCTREE)/tools/env \
+		-idirafter $(srctree)/tools/env \
 		-DUSE_HOSTCC \
 		-DTEXT_BASE=$(TEXT_BASE)