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.

config-portage.sh

@@ -7,3 +7,8 @@ set -e
 
 cp -r host-portage/. /etc/portage/
 cp -r portage/. /usr/${target}/etc/portage/
+
+# Set the Portage profile
+ln -snf \
+    /var/db/repos/gentoo/profiles/${profile} \
+    /usr/${target}/etc/portage/make.profile