board: freescale: p1_p2_rdb_pc: Move boot reset macros to p1_p2_bootsrc.h

board: freescale: p1_p2_rdb_pc: Move boot reset macros to p1_p2_bootsrc.h

Code for changing boot source is platform generic and can be used by any
P1* and P2* compatible RDB board. Not only by boards which use config
header file p1_p2_rdb_pc.h.

So move this code from p1_p2_rdb_pc.h to p1_p2_bootsrc.h and cleanup macros
for generating boot source env variables in CONFIG_EXTRA_ENV_SETTINGS.

This allows to use code for resetting board and rebooting to other boot
source also by other boards in future.

Signed-off-by: Pali Rohár <pali@kernel.org>