build: fix git prereq check to not rely on being in a git directory (#21968)

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

SVN-Revision: 48926

include/prereq-build.mk

@@ -155,7 +155,7 @@ $(eval $(call SetupHostCommand,svn,Please install the Subversion client, \
 	svn --version | grep Subversion))
 
 $(eval $(call SetupHostCommand,git,Please install Git (git-core) >= 1.7.12.2, \
-	git submodule update --help 2>&1 | grep -- --recursive))
+	git submodule --help 2>&1 | grep -q -- --recursive))
 
 $(eval $(call SetupHostCommand,file,Please install the 'file' package, \
 	file --version 2>&1 | grep file))