init: remount to tmpfs

runsc needs to be able to pivot_root. According to @lorenz this does not
work from initramfs. This introduces a temporary fix to re-mount and
re-exec into a new root based on tmpfs.

A proper fix would be to use a real filesystem instead of initramfs
(like squashfs), but this will do for now.

We also use this opportunity to use devtmpfs instead of manually
managing /dev. This collides with the storage manager that tries to
create all storage nodes - we just remove that.

Test Plan: shouldn't change behaviour

X-Origin-Diff: phab/D433
GitOrigin-RevId: aa59fec6551bab1b1b9c2fe037dce410e550981b
diff --git a/third_party/linux/linux-smalltown.config b/third_party/linux/linux-smalltown.config
index e2fcdf3..86d02a4 100644
--- a/third_party/linux/linux-smalltown.config
+++ b/third_party/linux/linux-smalltown.config
@@ -2542,7 +2542,7 @@
 # CONFIG_PROC_CHILDREN is not set
 CONFIG_KERNFS=y
 CONFIG_SYSFS=y
-# CONFIG_TMPFS is not set
+CONFIG_TMPFS=y
 # CONFIG_HUGETLBFS is not set
 CONFIG_CONFIGFS_FS=y
 CONFIG_EFIVAR_FS=y