build_torcx_store: Update for newer portage versions

Since EAPI=7 was supported, portage can no longer use different
ROOT and SYSROOT values.  The torcx images were installed into a
temporary root directory after being built using the board's
development files.  To continue using this setup, the torcx image's
packages are built as normal binary packages for the board root
without being installed, then the binary packages are installed in
the temporary torcx root.

build_torcx_store

@@ -79,12 +79,25 @@ function torcx_build() (
         [ -s "${tmproot}/etc/portage/bashrc" ] &&
         . "${tmproot}/etc/portage/bashrc"
 
+        # Build binary packages using dev files in the board root.
+        emerge-${BOARD} \
+            --buildpkg \
+            --buildpkgonly \
+            --nodeps \
+            --oneshot \
+            --quiet \
+            --root-deps=rdeps \
+            "${pkg}"
+
+        # Install the binary packages in the temporary torcx image root.
         emerge-${BOARD} \
             --nodeps \
             --oneshot \
+            --quiet \
             --root="${tmproot}" \
             --root-deps=rdeps \
-            --quiet \
+            --sysroot="${tmproot}" \
+            --usepkgonly \
             "${pkg}"
 )