Recognize EFI platform even in case of mismatch between Linux and EFI.

Recognize EFI platform even in case of mismatch between Linux and EFI.

Some x86 systems might be capable of running a 64-bit Linux kernel but
only use a 32-bit EFI (e.g. Intel Bay Trail systems). It's useful for
grub-install to be able to recognise such systems, to set the default
x86 platform correctly.

To allow grub-install to know the size of the firmware rather than
just the size of the kernel, there is now an extra EFI sysfs file to
describe the underlying firmware. Read that if possible, otherwise
fall back to the kernel type as before.

Signed-off-by: Steve McIntyre <steve@einval.com>