scripts: dump-target-info print kernel versions

This commits adds the ability to print Kernel versions of all
targets/subtargets. If a testing Kernel is set print that version as
well.

Example output:

    apm821xx/nand 5.10
    apm821xx/sata 5.10
    arc770/generic 5.4
    archs38/generic 5.4
    armvirt/32 5.10
    armvirt/64 5.10
    at91/sam9x 5.10
    at91/sama5 5.10
    ath25/generic 5.4
    ath79/generic 5.4 5.10
    ath79/mikrotik 5.4 5.10
    --- %< ---

This should help to get a quick update on the state of Kernels.

Signed-off-by: Paul Spooren <mail@aparcar.org>