_crossdev_ sets `ACCEPT_KEYWORDS="${ARCH} ~${ARCH}" by default, even
when run with `--stable`. This can cause conflicts when the host system
does not accept ~arch, and may not be desirable anyway. Projects that
want to use ~arch can set it in their own `make.conf`.
The _u-boot_ package does not have any stable keywords, so we have to
explicitly accept it.
Dustin C. Hatch
0f32bf3fcb