Drop bogus BOOTFLAG_* definitions

There is no code anywhere that references BOOTFLAG_* so remove these
defines.

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Heiko Schocher <hs@denx.de>
Cc: Valentin Longchamp <valentin.longchamp@keymile.com>
Cc: Peter Tyser <ptyser@xes-inc.com>