ARM: rpi_b: rework boot scripts, enable sysboot

Rework rpi_b's bootcmd (and sub-commands) to match Tegra's bootcmd as
much as possible. This will aid in a future patch which will create a
common header e.g. config_distro_bootcmd.h.

While at it, enable booting from extlinux.conf using the sysboot command.

The iteration and componentization currently makes a little more sense
for Tegra than RPi, but I'd still like to keep the two as similar as
possible. USB and networking support is coming to the RPi in due course,
and it'll all make more sense then anyway.

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>