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.

post-build.sh

@@ -5,10 +5,6 @@ set -e
 
 . "${CONFIGDIR:=${PWD}}"/config
 
-cp -uv \
-    /usr/${target}/boot/*.bin \
-    /usr/${target}/boot/*.dat \
-    /usr/${target}/boot/*.elf \
-    /usr/${target}/usr/share/u-boot/u-boot.bin \
-    config.txt \
-    "$1"/efi-part/
+if [ -f "${CONFIGDIR}"/post-build.sh ]; then
+    . "${CONFIGDIR}"/post-build.sh
+fi