Improve handling of circular dependencies

Several packages end up with circular dependencies, depending on which
Portage profile is selected.  The default profiles have a circular
dependency between *sys-libs/pam* and *sys-libs/libcap*.  Systemd and
SELinux profiles have even more issues.

We can break the circular dependencies by explicitly building *libcap*
with`USE=-pam` first, which happens to be the default configuration
generated by `crossdev`.  Then, we need to switch to a more complete
profile in order to build *glibc* and *util-linux*.  At this point, the
build root should be complete enough to build anything without circular
dependencies.

build.sh

@@ -5,10 +5,7 @@ set -e
 
 . ./config
 
-USE=-pam \
-${target}-emerge -1nvj --nodeps sys-libs/libcap
-USE='-cramfs -ncurses -nls -pam -readline -su -suid -systemd -udev' \
-${target}-emerge -1nvj --nodeps sys-apps/util-linux
+${target}-emerge -vuUDj sys-apps/util-linux
 
 ${target}-emerge -vnuUDj \
     net-misc/openssh \