build system: don't run xcompile or git for %clean/%config targets

It takes a long time for no gain: We don't need to update the
submodules, we don't need to fetch the revision, we don't need to find
the compilers, when all we want to do is to manipulate the .config file
or clean the build directory.

Change-Id: Ie1bd446a0d49a81e3cccdb56fe2c43ffd83b6c98
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Reviewed-on: https://review.coreboot.org/18182
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Martin Roth <martinroth@google.com>

Makefile.inc

@@ -13,7 +13,11 @@
 ## GNU General Public License for more details.
 ##
 
+ifneq ($(NOCOMPILE),1)
 GIT:=$(shell [ -e "$(top)/.git" ] && command -v git)
+else
+GIT:=
+endif
 
 #######################################################################
 # normalize Kconfig variables in a central place