misc-scripts: Add cp-full-kexts-to-basesystem.sh

Take a Big Sur installation, mount a read-write copy of BaseSystem.dmg,
and this script will replace each kext in /System/Library/Extensions
on the BaseSystem with a full copy from the running Big Sur.

When using this script, watch out for disk space on the BaseSystem.

misc-scripts/cp-full-kexts-to-basesystem.sh

@@ -0,0 +1,39 @@
+#!/bin/bash
+# For each kext that exists in /System/Library/Extensions on the
+# macOS Base System, replace it with a copy of the corresponding kext
+# from the running (Big Sur) system. Be careful; this script may have bugs,
+# particularly if macOS Base System is running low on disk space.
+# 
+# Assumption: read-write BaseSystem.dmg is already mounted
+
+if [ ! -d "/Volumes/macOS Base System" ]
+then
+    echo "Make sure read-write BaseSystem is mounted!"
+    exit 1
+fi
+
+# This seems to be necessary, at least sometimes, even though the image
+# is already in read-write format.
+mount -uw "/Volumes/macOS Base System" || exit 1
+
+# Delete the KernelCollections because we need the disk space!
+echo 'Deleting KernelCollections on macOS Base System.'
+rm -f "/Volumes/macOS Base System/System/Library/KernelCollections/"*.kc
+
+cd "/Volumes/macOS Base System/System/Library/Extensions" || exit 1
+for x in *
+do
+    # This reuses the same line of console output repeatedly
+    # so progress is visible without printing 400+ lines
+    # (but errors from cp will still be visible)
+    echo -n "Removing old $x..."
+    rm -rf "$x"
+    echo -ne "\033[2K" ; printf "\r"
+
+    echo -n "Copying new $x..."
+    cp -r "/System/Library/Extensions/$x" "$x"
+    echo -ne "\033[2K" ; printf "\r"
+done
+
+chmod -R 755 "/Volumes/macOS Base System/System/Library/Extensions"
+chown -R 0:0 "/Volumes/macOS Base System/System/Library/Extensions"