ARM: prevent compiler warnings from bootm.c

Without preceding declarations, "make C=1" generates
"Should it be static?" warnings for symbols
	do_bootm_linux,
	boot_prep_vxworks, and
	boot_jump_vxworks

Include of bootm.h also identified a signature mismatch
(const on argv[]).

Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com>
Acked-by: Albert ARIBAUD <albert.u.boot@aribaud.net>