msm: camera: config: Fix lahaina configuration

Top-level Makefile doesn't export techpack configs.
As such, we must revert to generating our own conf.h file instead of
relying on autoconf.h. This has the side-effect of not allowing
IS_REACHABLE/IS_ENABLED macro's from working on configs enabled within
a techpack.

CRs-Fixed: 2584631
Change-Id: I9c35b305b81c5a6a6892ca87ace9f69192fcca5b
Signed-off-by: Karthik Jayakumar <kjayakum@codeaurora.org>