Trigger mkinitfs_postprocess when generating legacy U-Boot images.

Trigger mkinitfs_postprocess script during legacy U-Boot generation the same way it's triggered in boot.img flow - right after the core blobs have been generated.

I'm preparing to submit an initial device port for Kindle 4, and I have need of this. That device has a hacked up U-Boot that expects to load a legacy U-Boot kernel image from a specific location in flash. So I want to kinda uh ... spruce up the generated U-Boot image a little bit - by strapping the postmarketOS initramfs to the end of it. It's all very proper and above board, I assure you.