install_cross_libs: fix toolchain dependency installation

`./setup_board --nousepkg --nogetbinpkg` currently fails with a
circular dependency due to pulling in the whole systemd-cryptsetup-udev
dependency chain. This is due to several issue:

* `emerge --root=$ROOT --emptytree` considers ROOT=/ to also be empty,
  so it pulls in all host packages. This must've not always been the case.
  So we need to pipe the dependency package list through `egrep $ROOT`
  to filter only those that would get installed into the desired ROOT
* if SYSROOT=/ and not SYSROOT=ROOT, then virtual/os-headers is missing
  from $ROOT package list
* the final filter expression tries to previously looked like this:
  (=sys-devel/gcc|sys-devel/binutils-0.9) which also matches
  sys-devel/gcc-config and sys-devel/binutils-config, which are
  necessary dependencies. Rework the match expression to not filter
  those out.

build_library/toolchain_util.sh

@@ -262,8 +262,9 @@ _get_dependency_list() {
 
     PORTAGE_CONFIGROOT="$ROOT" emerge "$@" --pretend \
         --emptytree --root-deps=rdeps --onlydeps --quiet | \
+        egrep "$ROOT" |
         sed -e 's/[^]]*\] \([^ :]*\).*/=\1/' |
-        egrep -v "(=$(echo "${pkgs[*]}")-[0-9])"
+        egrep -v "=($(echo "${pkgs[*]}"))-[0-9]"
 }
 
 # Configure a new ROOT
@@ -392,7 +393,7 @@ install_cross_libs() {
     # In order to get a dependency list we must calculate it before
     # updating package.provided. Otherwise portage will no-op.
     $sudo rm -f "${package_provided}/cross-${cross_chost}"
-    local cross_deps=$(ROOT="$ROOT" _get_dependency_list \
+    local cross_deps=$(ROOT="$ROOT" SYSROOT="$ROOT" _get_dependency_list \
         "$@" "${TOOLCHAIN_PKGS[@]}" | $sudo tee \
         "$ROOT/etc/portage/cross-${cross_chost}-depends")