flash-kernel: Add root kernel cmdline parameter

With flash-kernel, it's the OS responsibility to construct the kernel command line, including pointing the root partition.
3 jobs for flash-kernel in 3 minutes and 23 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Build
passed build:deb #8889
librem5

00:01:30

passed build:docs #8888
librem5

00:01:57

 
  Test
passed test:download #8890
librem5

00:01:25