fastboot driver: Allow colon in partition name when flashing

If fastboot flash vendor_boot:default, only
call has-slot on the first token, and append slot
to the first token only.

Test: fastboot flash vendor_boot:default
Bug: 173654501
Change-Id: I8ff7b3a0bafb964792ab8a788d64d14bc47ae83d
1 file changed