Add legacy_boot flag support for arm64 recipes

Some non-EFI arm64 platforms require the 'legacy_boot' flag to be set on the boot/root partition to boot on GPT, which is the default disk label defined by partman-partitioning for arm64.

Currently the '$bootable{ }' specifier sets the 'boot' flag, but on GPT this flag is related with the 'esp' flag which sets the 'EFI System Partition' type GUID.

So ignore '$bootable{ }' on GPT, introduce a new specifier '$legacy_boot{ }' which sets the 'legacy_boot' flag on GPT and add it to the /boot partition in arm64 recipes.

Closes: #1086051