Support the EFI Stub loader's splash image feature.

Support the EFI Stub loader's splash image feature.

Checks if `uefi_splash_image` exists in `dracutsysroot` if not unset
`uefi_splash_image`. Alternate Value parameter expansion adds section-vma
for splash image to EFI stub loader when the path to image is valid and
not an empty file.

I did not test on other distributions, but on Arch Linux the `systemd`
package includes a splash image at the path
`/usr/share/systemd/bootctl/splash-arch.bmp`. Perhaps, if this is a
common practice, a default image could be gathered from that directory.

It is required that the image be in bitmap (`.bmp`) format according to
`splash.c`.

The code for `stub.c` and `splash.c` can be found at:
https://github.com/systemd/systemd/blob/master/src/boot/efi/stub.c
https://github.com/systemd/systemd/blob/master/src/boot/efi/splash.c