kbuild: re-enable Makefile generation in a new O=... directory

The commit:
18c32dac75b187d1a4e858f3cfdf03e844129f5e "kbuild: fix
building with O=.. options"
disabled the creation of a Makefile in a new O=... directory. Restore it.

Signed-off-by: Guillaume Chazarain <guichaz@yahoo.fr>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

scripts/mkmakefile

@@ -13,7 +13,7 @@
 test ! -r $2/Makefile -o -O $2/Makefile || exit 0
 # Only overwrite automatically generated Makefiles
 # (so we do not overwrite kernel Makefile)
-if ! grep -q Automatically $2/Makefile
+if test -e $2/Makefile && ! grep -q Automatically $2/Makefile
 then
 	exit 0
 fi