MIPS: Refactor boot and boot/compressed rules

Split out the arch/mips/boot/ and arch/mips/boot/compressed/ targets
into boot-y and bootz-y variables. This makes it slightly cleaner to add
new targets.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/5793/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>