board, boot, package: remove usage of startup.nsh in EFI partition

The startup.nsh file is useless to boot EFI payloads. We just need to
follow the naming detection specified in the UEFI spec.
The EFI payload need to be placed in the boot/efi folder in the EFI partition
and follow the architecture naming as described below:
32bit : bootia32.efi
x64 : bootx64.efi
aarch32 : bootarm.efi
aarch64 : bootaa64.efi

This naming is already right in the packages involved (systemd, grub2,
gummiboot), therefore we just need to drop the generation of the
startup.nsh file.

The usage of the startup.nsh in genimage is also dropped to avoid errors in
the image generation.

Signed-off-by: Kory Maincent <kory.maincent@bootlin.com>
Tested-by: Erico Nunes <nunes.erico@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>