MIPS: Refactor load/entry address calculations

MIPS: Refactor load/entry address calculations

The vmlinux load address and entry address is calculated in multiple
places:
 - arch/mips/Makefile defines load-y from CONFIG_PHYSICAL_START (or
   defined by the platform) and passes it to
   arch/mips/boot/compressed/Makefile.
 - arch/mips/boot/compressed/Makefile calculates kernel entry using nm.
 - arch/mips/lasat/image/Makefile calculates both load and entry address
   using nm.

Lets combine these in the main Makefile and then pass them as Make
parameters to each of the three boot image Makefiles (in boot/,
boot/compressed, lasat/image/). The boot/ Makefile doesn't currently use
them, but will soon need to for U-Boot image targets.

The existing load-y definition is used in preference to calculating the
load address using nm.

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/5794/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>