Implement UEFI executable generation in mkinitcpio by utilizing UEFI
stubs provided by systemd/gummiboot.

This allows us to create a unified boot image we can boot from UEFI
with. These are practical for secure boot as we can sign initramfs,
kernel cmdline and the kernel all at once. By utilizing the
BOOT_LOADER_SPECIFICATION we can also drop new images into the correct
patch and have systemd-boot/gummiboot pick up the images.

The code does several things and does a fair amount of guessing to
figure out all the inputs needed.

We use `/etc/kernel/cmdline` to localize the kernel cmdline options we
want for the image. This is inherited from the `kernel-install` hook
system which might double as some form of standard.

We also do a dance to get the correct kernel image. We do a lookup into
/lib/modules and /boot for both versioned and unversioned kernels
(mainly Arch Linux).

There is an attempt to support both 32bit and 64bit lookup paths for the
stub images, but only 64bit is tested.

Gummiboot is also not tested.

https://www.freedesktop.org/software/systemd/man/kernel-install.html

https://systemd.io/BOOT_LOADER_SPECIFICATION/#type-2-efi-unified-kernel-images

Signed-off-by: Morten Linderud <morten@linderud.pw>

Signed-off-by: Morten Linderud <morten@linderud.pw>

Signed-off-by: Morten Linderud <morten@linderud.pw>

Signed-off-by: Morten Linderud <morten@linderud.pw>

Signed-off-by: Morten Linderud <morten@linderud.pw>

Signed-off-by: Morten Linderud <morten@linderud.pw>

Signed-off-by: Morten Linderud <morten@linderud.pw>

Signed-off-by: Morten Linderud <morten@linderud.pw>

Signed-off-by: Morten Linderud <morten@linderud.pw>

Signed-off-by: Morten Linderud <morten@linderud.pw>

Preferably we shouldn't need to have a dry run when `mkinitcpio -U` is
invoked, instead create a temporary initramfs we use for the UEFI
executable.

Signed-off-by: Morten Linderud <morten@linderud.pw>

Signed-off-by: Morten Linderud <morten@linderud.pw>