Mention BSP target names in README
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 <ab3b2ee9094bbfcde188a022dd40abb8fbbe94dd@bytesatwork.ch>

bytesatwork/meta-bytesatwork