Mention BSP target names in README

Mention the names for Linux Kernel and U-Boot in the README so it's
easier for a user.

Signed-off-by: Urs Fässler <urs.fassler@bytesatwork.ch>
Remove minimal-image and devfull-image

There's no use for these images.

Signed-off-by: Urs Fässler <urs.fassler@bytesatwork.ch>