brcm47xx: rename target to bcm47xx

This change makes the names of Broadcom targets consistent by using
the common notation based on SoC/CPU ID (which is used internally
anyway), bcmXXXX instead of brcmXXXX.
This is even used for target TITLE in make menuconfig already,
only the short target name used brcm so far.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>