prepare: Ignore ACCEPT_KEYWORDS in crossdev.conf

_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.

portage/package.accept_keywords/uboot

@@ -0,0 +1 @@
+sys-boot/u-boot ~arm64

prepare.sh

@@ -37,6 +37,7 @@ if [ ! -f portage/make.conf/10-crossdev.conf ]; then
     sed -r \
         -e 's: ?-pam::' \
         -e '/PKGDIR=/d' \
+        -e '/ACCEPT_KEYWORDS/d' \
         /usr/${target}/etc/portage/make.conf \
         > "${O}"/portage/etc/portage/make.conf/10-crossdev.conf
 fi