feat(fittool): Allow to initialize FIT by defining an offset
feat(fittool): Allow to initialize FIT by defining an offset

Previously "fittool init" required to provide pointer value,
which is sometimes difficult to calculate. Providing a more
handy option to pass an offset instead (so that the pointer
will be calculated automatically).

    $ dd if=/dev/zero of=/tmp/test bs=1K count=64
    64+0 records in
    64+0 records out
    65536 bytes (66 kB, 64 KiB) copied, 0.000296419 s, 221 MB/s
    $ go run ./ init -f /tmp/test --pointer-from-offset $(( 63 * 1024 ))
    $ hexdump -C /tmp/test
    00000000  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
    *
    0000fc00  5f 46 49 54 5f 20 20 20  01 00 00 00 00 01 80 80  |_FIT_   ........|
    0000fc10  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
    *
    0000ffc0  00 fc ff ff 00 00 00 00  00 00 00 00 00 00 00 00  |................|
    0000ffd0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
    *
    00010000

Signed-off-by: Dmitrii Okunev <74d81ae1898d935b77a01158fdef590f9767e0f5@fb.com>

linuxboot/fiano