kbuild: PWD isn't set reliably by shell. srctree is the proper macro.

Current makefile is using a make macro of $(PWD) in shell command to
find the root of the kernel source tree, this has a problem when
building the kernel outside of the android build system as this is an
Android macro no a kbuild one.  We can improve this by using the
$(srctree) macro the kbuild system provides for exactly this purpose.

Change-Id: I0cb7611a6f770eb2651f642f12e51efcc2b9e729
Signed-off-by: PJ Bostley <pbostley@codeaurora.org>