grub-efi-native: Add support for EFI ISO images

grub-efi-native: Add support for EFI ISO images

The iso9660 file system support needs to be added to grub in order to
be able to correctly find the grub.cfg.  The grub commands to locate
the grub.cfg also needs to be encoded into grub's default
configuration.

This change allows the resulting grub binary to work both in the hard
drive / USB boot case or the optical media boot case.

[YOCTO #4100]

(From OE-Core rev: 0bc0762a479b5182a07fccb2b1e9cd5fc15ca485)

Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
Reviewed-by: Darren Hart <dvhart@linux.intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>