image-buildr: Fixup bootargs for luks boot

This works around librem5-devkit-base putting an
unencrypted root device into flash-kernel's boot
args.

Librem5/librem5-devkit-tools#32
1 job for luks in 10 seconds (queued for 18 minutes and 1 second)
Status Job ID Name Coverage
  Test
passed #312336
librem5
job

00:00:10