MIPS: Clean up the calculation of VMLINUZ_LOAD_ADDRESS

We have calculated VMLINUZ_LOAD_ADDRESS in shell, which is indecipherable. This
patch rewrites it in C.

Signed-off-by: Wu Zhangjin <wuzhangjin@gmail.com>
To: linux-mips <linux-mips@linux-mips.org>
Cc: Alexander Clouter <alex@digriz.org.uk>
Cc: Manuel Lauss <manuel.lauss@gmail.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Alexander Clouter <alex@digriz.org.uk>
Patchwork: https://patchwork.linux-mips.org/patch/1324/


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