bootstrap: allow calling genmf with non default shell

(cherry picked from commit 6fa1935b5a4d1894689fac30c358a5b1887677c9)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

bootstrap

@@ -78,7 +78,7 @@ echo "generating modules/**/Makefile.am"
 find modules/ -name Modules.am | \
 sed -ne 's,modules/\(.*\)/Modules.am,\1,p' | \
 while read d; do
-	sh modules/genmf "$d"
+	${CONFIG_SHELL-sh} modules/genmf "$d"
 	printf "."
 done
 printf "\n"