pinephone: implement boot-fail behavior in u-boot

Currently when the boot fails due to whatever reason (and you get dropped to a u-boot shell) there's absolutely zero indication to the user that the phone is on and that the boot has failed. Something should be implemented there so the user knows what's happening (without having a serial console plugged into the phone)

e.g. in this case:

Hit any key to stop autoboot:  0

Device 0: unknown device
phy interface7
mdio_register: non unique device name 'ethernet@1c30000'
Could not get PHY for ethernet@1c30000: addr 1
phy interface7
mdio_register: non unique device name 'ethernet@1c30000'
Could not get PHY for ethernet@1c30000: addr 1
phy interface7
mdio_register: non unique device name 'ethernet@1c30000'
Could not get PHY for ethernet@1c30000: addr 1
phy interface7
mdio_register: non unique device name 'ethernet@1c30000'

...

phy interface7
mdio_register: non unique device name 'ethernet@1c30000'
Could not get PHY for ethernet@1c30000: addr 1
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