u-boot: Be mindful of other BSPs when it comes to u-boot overrides
We are sort of hijacking OE-Core recipe here, so lets be careful and
make it such that it only happens for risv64, which makes a multi-BSP
scenario work again.
This repo seems to be rebasing :( so add rebaseable=1 so bitbake knows
about it
update to latest rev as of today
Signed-off-by: Khem Raj <raj.khem@gmail.com>