payloads/external/LinuxBoot: Adopt u-root change

The u-root toolkit dropped the original uinit bootloader in the master
branch and respectively the systemboot-option in templates.go. In
consequence the LinuxBoot builds will boot into the u-root shell and
waiting for input. This commit enables the reuse of the -uinitcmd flag
to specify a command, which runs after the u-root init-process.

Systemboot as a bootloader wrapper will mimic a BIOS/UEFI boot device
selection. Other preselections, i.e. stboot and boot2 are implemented as
well. Custom strings or programs can be set as well, but they cannot
contain program flags. E.g. 'fbnetboot -class linuxboot' will not work,
because they aren't symlinkable.

This commit and its respective LinuxBoot builds with systemboot, none
and one custom option have been tested successfully on a UP squared
single board computer with the intended behavior.

Change-Id: I4ac3409040ea77a1836f90f43fba07d2cd05a952
Signed-off-by: Patrik Tesarik <mail@patrik-tesarik.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/40316
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Christian Walter <christian.walter@9elements.com>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>

payloads/external/LinuxBoot/targets/u-root.mk

@@ -15,6 +15,7 @@ go_version_minor=$(shell echo $(go_version) |  sed -nr 's/^([0-9]+)\.([0-9]+)\.?
 
 uroot_args+=-build=$(CONFIG_LINUXBOOT_UROOT_FORMAT)
 uroot_args+=-initcmd $(CONFIG_LINUXBOOT_UROOT_INITCMD)
+uroot_args+=-uinitcmd=$(CONFIG_LINUXBOOT_UROOT_UINITCMD)
 uroot_args+=-defaultsh $(CONFIG_LINUXBOOT_UROOT_SHELL)
 ifneq (CONFIG_LINUXBOOT_UROOT_FILES,)
 uroot_args+=$(foreach file,$(CONFIG_LINUXBOOT_UROOT_FILES),-files $(PWD)/$(file))