[build] Set GITVERSION only if there is a git repository

The $(BIN)/version.%.o target will fail if iPXE is built within a
non-git repository, e.g. when the user downloaded and extracted an
archive containing iPXE sources, *and* if any parent directory of the
iPXE sources is a git repository (or even contains a directory named
".git").  This is because git will by default ascend the directory
tree and look for ".git".

The problem typically manifests on source based distributions, see for
example https://bugs.gentoo.org/show_bug.cgi?id=482804

Modified-by: Michael Brown <mcb30@ipxe.org>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/Makefile

 EXTRAVERSION	= +
 MM_VERSION	= $(VERSION_MAJOR).$(VERSION_MINOR)
 VERSION		= $(MM_VERSION).$(VERSION_PATCH)$(EXTRAVERSION)
+ifneq ($(wildcard ../.git),)
 GITVERSION := $(shell git describe --always --abbrev=1 --match "" 2>/dev/null)
-ifneq ($(GITVERSION),)
 VERSION		+= ($(GITVERSION))
 endif
 version :