Add new package for building bootloader for the SiFive U-series boards. Supported
boards at this stage are the HiFive Unleashed and HiFive Unmatched.
Signed-off-by: Zoltan HERPAI <wigyori@uid0.hu>
(cherry picked from commit 91406797f9d06c0008f0a8c2c8455abfb37bf28c)