librem5: Enable distro_bootcmd

This enables distro_bootcmd for the imx8m SOM and the Librem5 dev kit.
While the distro protocol is not fully adhered to, the boot.scr from flash-kernel 2db0e4a3 can be booted.
Status Job ID Name Coverage
  Build
failed #7863
build

 
  Test
skipped #7865
allowed to fail
code_quality
skipped #7864
test
 
Name Stage Failure
failed
build Build There has been a timeout failure or the job got stuck. Check your timeout limits or try again
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