Make it easy to export system.img and boot.img outside of the chroots

Created by: ollieparanoid

From issue #73 (closed).

@alexlep:

I was using SP flash tool to flash images, and at first I created boot.img manually using mkbootimg from aports. Maybe we could do smth like: ./pmbootstrap.py build bootimage which will build mkbootimg, install it to chroot, and automatically will build boot.img with kernel/ram files and params from deviceinfo?

@MartijnBraam:

I was thinking of something like pmbootstrap export [path] to export boot.img and system.img to a specific path.