board/BuR/brppt1: implement more flexible boot process

board/BuR/brppt1: implement more flexible boot process

With this commit we do:

- set the bootdelay in all brppt1 defconfigs to 0, this makes
development easier, since we can break into serial console.

- move CONFIG_BOOTCOMMAND from header file to defconfig

- introduce b_mode variable for selecting the final boot-target.
This b_mode represents the boot-switch, which can found on most b&r
targets. On the brppt1 this boot-switch is derived from some gpio and
the bootcounter within the RTC block, making it so possible to force
a boot-target (as example for repair-case).

- refactor the environment for booting new flexible way
primary we want to get some bootscr.img within the mass-storage,
this script then loads everything needed for the boot.
For legacy reason we implement the t30lgcy#x boot targets, booting the
already delivered linux-images.

- make space for the cfgscr within mtdparts on brppt1_nand

Signed-off-by: Hannes Schmelzer <oe5hpm@oevsv.at>