crossgcc: Clean GNU make context so builds succeed

GCC's build system is sometimes confused by our build system's
configuration: make crossgcc failed, while
util/crossgcc/buildgcc -p armv7-a-eabi didn't.

Make sure the GCC build system runs independently from
ours by breaking any ties.

Change-Id: I563e17b22127bc8c83ebfb17252184a3b6e0e58b
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/8545
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
diff --git a/util/crossgcc/buildgcc b/util/crossgcc/buildgcc
index e4ddbc2..0779266 100755
--- a/util/crossgcc/buildgcc
+++ b/util/crossgcc/buildgcc
@@ -64,6 +64,8 @@
 PYTHON_DIR="Python-${PYTHON_VERSION}"
 EXPAT_DIR="expat-${EXPAT_VERSION}"
 
+unset MAKELEVEL MAKEFLAGS
+
 SAVETEMPS=0
 SKIPGDB=1
 SKIPPYTHON=1