fastboot: [boot] and [flash:raw] command support boot v4

Boot v4 is mostly compatible with boot v3, so these minimal changes
should be sufficient.
The newly introduced field of v4, signature_size, is left zero because
this field is only meant to be read by VTS testcases, so the value
doesn't matter for the debugging purposes of boot and flash:raw.

Bug: 183455415
Test: Manual
Change-Id: I5c2737ce35dd25f318b19a429de805ea16c46607