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 <email@example.com>