We now use MIPS_CMDLINE_DTB_EXTEND for all boot varieties
(tftpboot, flash boot, kexec) with the addition of
MIPS_BOOTLOADER_CMDLINE_REQUIRE_COOKIE - local patch -
so that the bootloader args are ignored unless they
contain the string "liminix"
This allows booting a new image from a running OS, creating a
phram mtd for the root squashfs
* enable CONFIG_KEXEC
* add modules/kexecboot
* patch kexec-tools to add --map-file option for the squashfs
* patch kernel kexec code to call new kernel with DTB
by having two separate derivations for patching the kernel source tree
and building it, we have to copy said source trees from one store
location to another which takes non-neglible time on spinning rust
(literally minutes on my machine). Replace with a single derivation
that can do more things on one tree in-place
