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/config.txt

@@ -0,0 +1,17 @@
+arm_64bit=1
+
+start_x=1
+
+bootcode_delay=0
+boot_delay=0
+
+gpu_mem=32
+
+kernel=u-boot.bin
+
+enable_uart=1
+dtoverlay=miniuart-bt
+
+dtparam i2c_arm=on
+
+device_tree=bcm2711-rpi-cm4-ha-yellow.dtb