google-{asurada,cherry,corsola,gru,kukui,oak,trogdor}: enable depthcharge image compression

Enable compression to reduce the size of vmlinuz.kpart. This fixes the regular initramfs being too big to fit in kukui's small kernel partition size (the bootloader supports larger partitions but changing the size in the device package would break old installs). I enabled this for every supported device to prevent running into more size issues, but some like asurada would most likely never reach the partition size limit (128MB). I am unsure how much this impacts boot times, but google uses this so it shouldn't be too bad.

Tested devices:

• Asurada
• Cherry
• Corsola
• Gru
• Kukui
• Oak
• Trogdor

I'm looking for testers for Cherry, Corsola, Gru, and Oak. The rest I can test myself.

Depends on !5373 (merged)