Use environment variable to find unifdef tool

Tools used within the sandbox are now copied into the
sandbox, see aosp/1531944. This caused the modified
headers_install.sh, which is no longer installed, to
point to a non-existent location.

This change adds a level of indirection. The
gen-headers_install.sh module no longer uses unifdef as
a tool, but still modifies the headers_install.sh
script, but not to point to a particular location, but
to find the unifdef tool via an environment variable,
LOC_UNIFDEF.

Next, we modify qti_generate_kernel_headers_arm and
qti_generate_kernel_headers_arm64 to need the unifdef
tool (which is copied into the sandbox for these tools).

We add a new --unifdef option to the kernel_headers.py
script so that it can find the tool in the sandbox. The
kernel_headers.py script sets the LOC_UNIFDEF
environment variable before invoking the altered
headers_install.sh script (also copied into the
sandbox).

Finally, we generate gen_headers_arm.bp and
gen_headers_arm64.bp with all of these changes.

Bug: 178500203
Change-Id: Ie3b8c36b7d60bd950c28bac566e04f43de78cf98
Signed-off-by: Mohammed Athar <athar@codeaurora.org>
Signed-off-by: Shadab Naseem <snaseem@codeaurora.org>
Signed-off-by: Srinivasarao P <spathi@codeaurora.org>