pmb/config: enable "flasher boot" for fastboot-bootpart

Minecrellrequested to merge
Minecrell/pmbootstrap:fastboot-bootpart-boot into master

Devices using fastboot-bootpart typically boot directly from the boot file system. They might still support "fastboot boot" though, which can be used to boot a modified kernel more easily. Enable this similar to the existing "flash_boot" action.

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