payloads: Add LinuxBoot payload in u-root mode

* Add LinuxBoot support
* Add u-root mode
* Download kernel and u-root from upstream sources.
* Add customization options
* Clean kernel only if directory exists

Change-Id: I3a25ff6812e046acc688cbbb203cf262ad751659
Signed-off-by: Philipp Deppenwiese <zaolin@das-labor.org>
Reviewed-on: https://review.coreboot.org/23071
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
8 files changed