ANDROID: kbuild: fix modfinal with LTO

Makefile.modpost was split to Makefile.modfinal in 5.4. This file
doesn't include include/config/auto.conf, which breaks checking
for kernel configuration. This change adds the missing include and
cleans up the LTO build rule.

Bug: 145296861
Fixes: 310afefe ("ANDROID: kbuild: add support for Clang LTO")
Change-Id: I3e6f676e841eed730ce8cccdfbd312f63660c293
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>