ARM: shmobile: remove Makefile.boot

Now that the only multiplatform boot is supported for all
shmobile platforms Makefile.boot may be removed.

This seems correct to me given:
* Setting MACHINE to no value in arch/arm/Makefile
  if CONFIG_ARCH_MULTIPLATFORM is set
* Not including Makefile.boot in arch/arm/boot/Makefile
  if MACHINE has no value
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>