[efi] Allow currently selected image to be opened as "grub*.efi"

[efi] Allow currently selected image to be opened as "grub*.efi"

Versions 15.4 and earlier of the UEFI shim are incapable of correctly
parsing the command line in order to extract the second stage loader
filename, and will always attempt to load "grubx64.efi" or equivalent.

Versions 15.3 and later of the UEFI shim are currently incapable of
loading a Linux kernel directly anyway, since the kernel does not
include SBAT metadata.  These versions will require a genuine
shim-signed GRUB binary to be used as a crutch to assist shim in
loading a Linux kernel.

This leaves versions 15.2 and earlier of the UEFI shim (as currently
used in e.g. RHEL7) as being capable of directly loading a Linux
kernel, but incorrectly attempting to load it using the filename
"grubx64.efi" or equivalent.  To support the bugs in these older
versions of the UEFI shim, allow the currently selected image to be
opened via any filename of the form "grub*.efi".

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