payloads/external/LinuxBoot: Clean up

There were some issues with the current Linuxboot Makefiles.
- multithreaded compilation didn't work, because some prerequisites
  were missing
- initramfs wasn't added for x86 qemu boot.
- riscv support was incomplete

It began with separate patches, but resulted in a clean up patch, that
is hard to separate. The most important changes are the following:
- Instead of phony targets, actual files are now used as prerequisites
- riscv can now be used as target
- initramfs works now also for x86
- instead of querying the most recent version from the internet, I set a
  known working version (because I tested it) that can be customized
  and/or upgraded in the future. The reasons:
      - querying the version from the internet requires a constant
        connection to the internet even after linux kernel is already
        build (aka subsequent builds).
      - one usually wants to use a known working version, but optionally
        still have the posibillity to choose a custom one. This patch
        introduces this possibility in its most simple form.
- I removed as much ifeq statements as possible and moved that
  responsibility to Kconfig, because they tend to make the
  Makefile less readable.

Signed-off-by: Maximilian Brune <maximilian.brune@9elements.com>
Change-Id: I25e757108e0dd473969fe5a192ad0733f1fe6286
Reviewed-on: https://review.coreboot.org/c/coreboot/+/76150
Reviewed-by: Lean Sheng Tan <sheng.tan@9elements.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

0 files changed, 0 insertions, 0 deletions