efi: Make the gnu-efi build scripts more portable

Not all distributions point /bin/sh at /bin/bash, so remove some
bashisms (pushd/popd) and require that build-gnu-efi.sh be run from the
Syslinux object directory.

Also, swap realpath(1) for readlink(1) because the former isn't
available on Debian.

Reported-by: Ferenc Wagner <wferi@niif.hu>
Reported-by: Celelibi <celelibi@gmail.com>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>