BuildSystem: Add support for building initrd image

This is nice for testing when there isn't xhci controller available or
my usb stack fails :)

toolchain/grub-legacy-initrd.cfg

@@ -0,0 +1,4 @@
+menuentry "banan-os" {
+	multiboot2 /boot/banan-os.kernel readonly
+	module2 /boot/banan-os.initrd
+}

toolchain/grub-uefi-initrd.cfg

@@ -0,0 +1,11 @@
+insmod part_gpt
+insmod search_fs_uuid
+
+search --no-floppy --fs-uuid --set=root <ROOT_FS>
+
+insmod all_video
+
+menuentry "banan-os" {
+	multiboot2 /boot/banan-os.kernel readonly
+	module2 /boot/banan-os.initrd
+}