fastboot: Add a command to get current kernel logs.

This adds "fastboot getvar dmesg", usable only in userspace fastboot,
and only on unlocked devices. This is to debug flashing failures when
serial log access is not available.

Bug: 230269532
Test: fastboot getvar dmesg
Change-Id: Ic57881e362efe3f0687f41ab986d30b3a59dc4e4