boot/beaglev-secondboot: new package

This packages allows to build the first stage bootloader used on the
BeagleV, which is used even before the DDR initialization and
OpenSBI/U-Boot. Yes, "secondboot" is strange for what is the first
stage bootloader, but that's the upstream name.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
[yann.morin.1998@free.fr:
  - add hash file
  - commit is HEAD only right now, so don't reference HEAD
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>