Move MODULE_CFLAGS earlier in the list

Cheat, and make it secretly be MODULE_CPPFLAGS, since ~all callers
are using it to modify the header search list or define preprocessor
symbols.  As with per-file CPPFLAGS, this should be early in the
command line, so that they can add to the front of the include search
path.

Change-Id: I1ec883194a5be6f7d17e6b111ddac6ef97f88782