tizen: bootscript: rpi4: update boot scripts

tizen: bootscript: rpi4: update boot scripts

Update RPi4 boot script to match the one developed recently for
LicheePi4A/VF2. This improves the following things:
- switch to generic devtype/devnum envs like for typical distro_boot case
- correctly load script and kernel from boot_b
- root= kernel parameter is correctly set for partitions up to 19th
- reordered some operations and added more comments
- tested A/B booting
- tested booting from legacy MBR based layout
- tested USB booting

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Change-Id: If038c93db7fc7f8a6f009e5dd43ae74286a6edb2