BFS: Add support for building big-endian BFS

I tried to use the feature stuff to relocate objects but didn't managed
to get it to work, having another subdir seems to be the simplest
solution.

I managed to mount a clone of my BeBox' drive, but it KDLed shortly when mouting
read-write.

Change-Id: Ia4f126673e553e4f3e524a40218e6c623527b96d
Reviewed-on: https://review.haiku-os.org/c/645
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>