[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>
[ioapi] Centralise notion of PAGE_SIZE

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