Add support for more bootloaders

[ricardon]

Today we only use grub as bootloader. It might be good to test more bootloaders. For instance:

Linux as bootloader (aka EFI stub)
systemd boot

This could help to uncover issues as by the time Linux boots the bootloader has already interatected with the firmware (e.g., grub called by this point the UEFI Boot time service ExitBootServices()).

[sakar-arora]

At Arm, we are using EFI stub as the default mechanism to boot luvOS for running the enterprise architecture compliance suite. And it works smoothly.

[meghadey]

Add a distro like option to select which bootloader to run LUV on.
-eLiLo
-efi STUB
-systemd boot
-syslinux

For now, we can eliminate BITS for the above bootloaders.

[ricardon]

@meghadey suggested that a good quick test is to change the EFI_PROVIDER from "luv-efi" to "systemd-boot" here [1] and see how the boot partition is populated; or at least see what things break when building.

At a later stage we would need to wire things up to run BITS from systemd-boot, implement configurations for netconsole, tests suites to run, etc.

[1]. https://github.com/intel/luv-yocto/blob/master/meta-luv/conf/distro/luv.conf#L13