boot/riscv-pk: add bootloader for RISC-V architecture

boot/riscv-pk: add bootloader for RISC-V architecture

Add the RISC-V Proxy Kernel (pk) package which provides the Berkeley
Boot Loader for booting RISC-V kernel images.

Signed-off-by: Mark Corbin <mark.corbin@embecosm.com>
[Thomas:
 - fix prompt of option to be just "riscv-pk"
 - add blank line between help text and upstream URL in Config.in file
 - use $(INSTALL) instead of cp to install the generated image]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>