base-uboot: provide explicit earlycon support

Some boards use console_device to set their earlycon parameter, others
use extra_boot_args. extra_boot_args may already be used by a test
template, e.g. kernelci already uses that parameter. If you (mis-)use
console_device you have to make sure you put it before the actual device
string, because the baudrate and console parameters are appended. For
example you have to use "set console_device 'earlycon ttyS0'" and must
not use "set console_device 'ttyS0 earlycon'".

Make it easier for the user and provide a native method to set the
earlycon parameter through console_earlycon.

Signed-off-by: Michael Walle <michael@walle.cc>
16 jobs for fix-uboot-base in 16 minutes and 52 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Test
passed #117352
amd64
dispatcher-debian-10

00:03:57

passed #117354
amd64
dispatcher-debian-11

00:04:37

passed #117353
amd64
server-debian-10

00:04:36

passed #117355
amd64
server-debian-11

00:06:49

 
  Analyze
passed #117356
amd64-dind
bandit-sast

00:00:52

passed #117357
amd64
black

00:00:34

passed #117358
amd64
code_quality

00:00:20

passed #117363
amd64
codespell

00:00:18

passed #117359
amd64
coverage

00:06:28

60.86%
passed #117360
amd64
dockerfiles

00:00:17

passed #117362
amd64
pylint

00:03:03

passed #117361
amd64
schemas

00:00:30

 
  Build
passed #117365
amd64
debian/10

00:02:33

passed #117364
amd64
doc

00:01:23

passed #117366
amd64-dind
docker-amd64-dispatcher

00:02:39

passed #117367
amd64-dind
docker-amd64-server

00:03:32