Move post-build script to configdir

The base Aimee OS build does not need any post-installation tasks.
Custom builds can provide a `post-build.sh` script to implement the
tasks they need.  For example, builds targeting Raspberry Pi devices
can use this script to install the firmware files.

yellow/post-build.sh

@@ -0,0 +1,9 @@
+# vim: set ft=sh sw=4 ts=4 sts=4 et :
+
+cp -uv \
+    /usr/${target}/boot/*.bin \
+    /usr/${target}/boot/*.dat \
+    /usr/${target}/boot/*.elf \
+    /usr/${target}/usr/share/u-boot/u-boot.bin \
+    "${CONFIGDIR}"/config.txt \
+    "$1"/efi-part/