Android.mk: cleanse all set but not unset variables

Discovered by diffing the set of "set variables" with
the set of "cleared variables".

Script:

mydir=$(mktemp -d)

grep -E '(^[a-z].)[a-z0-9_\.]*\s*:?=.' Android.mk  | cut -d' ' -f 1-1 | sort | uniq > $mydir/set_vars
grep -E '(^[a-z].)[a-z0-9_\.]*\s*:?=$' Android.mk | cut -d' ' -f1-1 | sort | uniq > $mydir/unset_vars
diff $mydir/set_vars $mydir/unset_vars
rm -rf $mydir

Change-Id: Ib50abac6b417a1bcc1894d9a7bafdbdca371006a
Signed-off-by: William Roberts <william.c.roberts@intel.com>