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 Name Job ID Coverage
  Test
passed dispatcher-debian-10 #117352
amd64

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passed dispatcher-debian-11 #117354
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passed server-debian-10 #117353
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passed server-debian-11 #117355
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  Analyze
passed bandit-sast #117356
amd64-dind

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passed black #117357
amd64

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passed code_quality #117358
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passed codespell #117363
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passed coverage #117359
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60.86%
passed dockerfiles #117360
amd64

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passed pylint #117362
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passed schemas #117361
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  Build
passed debian/10 #117365
amd64

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passed doc #117364
amd64

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passed docker-amd64-dispatcher #117366
amd64-dind

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passed docker-amd64-server #117367
amd64-dind

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