Shuffle around the creation of sysb

For further RAM savings, we want to create sysb -without- having linux
tarball extracted at the same time. To accomplish this, we move the
gen_init_cpio + related script out of the tree and create it afterward.

Also use hard links to free up some more space.

sysa/linux-4.9.10/patches/initramfs_list.patch

@@ -0,0 +1,17 @@
+SPDX-FileCopyrightText: 2021 fosslinux <fosslinux@aussies.space>
+
+SPDX-License-Identifier: GPL-2.0-only
+
+Use gen_initramfs_list.sh from outside kernel source tree.
+
+--- scripts/gen_initramfs_list.sh	2021-10-10 20:37:12.220231015 +1100
++++ scripts/gen_initramfs_list.sh	2021-10-10 20:37:22.694296140 +1100
+@@ -310,7 +310,7 @@
+ 			fi
+ 		fi
+ 		cpio_tfile="$(mktemp ${TMPDIR:-/tmp}/cpiofile.XXXXXX)"
+-		usr/gen_init_cpio $timestamp ${cpio_list} > ${cpio_tfile}
++		gen_init_cpio $timestamp ${cpio_list} > ${cpio_tfile}
+ 	else
+ 		cpio_tfile=${cpio_file}
+ 	fi