stand/uboot: reorg

Build uboot ubldr and friends like we build efi binaries
o move everything to be under stand/uboot
o md code goes in arch/$ARCH
o move everything over from the library

• Had to rename console.c, disk.c and module.c due to conflicts

o update version to 1.5 to reflect the new way of building

This results in a more consistent build system and should represent no
functional change, apart from powerpc version getting new help
file. Also, moved to exlcuding uboot on powerpc64le by using
BROKEN_OPTION instead of the incidental exclusion we had before due to
Makefile reorgs.

Sponsored by: Netflix
Feedback by: stevek, jrtc27
Differential Revision: https://reviews.freebsd.org/D33362