Check the git command before setting TOPDIR

Since git doesn't necessarily exist in a build system, skip the git
command to avoid the command-not-found warning.

Signed-off-by: Gary Lin <glin@suse.com>

Makefile

@@ -13,7 +13,11 @@ EFI_ARCH ?= $(shell $(CC) -dumpmachine | cut -f1 -d- | \
 		)
 
 ifeq ($(MAKELEVEL),0)
+ifneq (, $(shell which git))
 TOPDIR != if [ "$$(git rev-parse --is-inside-work-tree)" = true ]; then echo $$(realpath ./$$(git rev-parse --show-cdup)) ; else echo $(PWD) ; fi
+else
+TOPDIR != echo $(PWD)
+endif
 BUILDDIR ?= $(TOPDIR)/build-$(EFI_ARCH)
 endif