module/u-root: added build parameter for uroot template

[flammit]

trafficstars

Instead of using UROOT_CMDS too specify the packages to include use UROOT_TEMPLATE=minimal or all, etc.

[hugelgupf]

Templates = cmds in u-root. Templates are just textual expansion, so UROOT_CMDS=minimal should work without this patch

[flammit]

As per slack, right now the Heads u-root module makefile appends the full package path to each command in UROOT_CMDS:

u-root_src_cmds := $(foreach cmd,$(UROOT_CMDS),github.com/u-root/u-root/cmds/$(cmd))

so we need another option to deliver unmodified arguments to the u-root build.

[tlaurion]

@hugelgupf : mergeable?

[tlaurion]

@flammit ?

[tlaurion]

@hugelgupf really unaware at this point if linuxboot can be even built on top of coreboot inside of Heads. Is it supposed to?