prepare: Update to modern Portage profile

The 17.0 profiles are deprecated.  Let's use the project's configured
profile at this stage, so as not to have to rebuild stuff because we
change USE flages, etc. once we set it later.

The modern profiles are all "merged-usr" by default now, so we do not
need that explicit step anymore.

prepare.sh

@@ -38,7 +38,7 @@ if [ ! -f portage/make.conf/10-crossdev.conf ]; then
         > "${CONFIGDIR}"/portage/target/etc/portage/make.conf/10-crossdev.conf
 fi
 
-ln -snf /var/db/repos/gentoo/profiles/default/linux/${ARCH}/17.0 \
+ln -snf /var/db/repos/gentoo/profiles/${profile} \
     /usr/${target}/etc/portage/make.profile
 
 FEATURES="${FEATURES}" \
@@ -47,14 +47,9 @@ ${target}-emerge -bk1nvj sys-libs/glibc
 
 FEATURES="${FEATURES}" \
 PKGDIR=$(portageq envvar PKGDIR) \
-USE=pam \
+USE='pam build' \
 ${target}-emerge -bknvj sys-apps/util-linux
 
-# Migrate to the merged-usr layout
-mkdir -p /usr/${target}/bin
-emerge -bknv --quiet-build=y merge-usr
-merge-usr --root=/usr/${target}
-
 # Set the Portage profile
 ln -snf \
     /var/db/repos/gentoo/profiles/${profile} \