Error on USE_CLANG_PLATFORM_BUILD=

A blank USE_CLANG_PLATFORM_BUILD= on the command line for make causes
confusion between make and kati.  make ignores writes to variables set
on the command line, so envsetup.mk cannot set it to true,
USE_CLANG_PLATFORM_BUILD is still empty in soong.mk, and soong.variables
gets DeviceUsesClang: false.  kati gets USE_CLANG_PLATFORM_BUILD= in its
environment instead of on its command line, so envsetup.mk when parsed
by kati sets USE_CLANG_PLATFORM_BUILD=true.

Error out early if USE_CLANG_PLATFORM_BUILD is not true or false after
attempting to set it to true if it is empty.

Change-Id: I0e2731315711db9a6ac6ea2bf49c88bccacd35f7