MIPS: Fix vmlinuz build for 32bit-only math shells

MIPS: Fix vmlinuz build for 32bit-only math shells

POSIX requires $((<expression>)) arithmetic in sh only to have long
arithmetic so on 32-bit sh binaries might do only 32-bit arithmetic but
the arithmetic done in arch/mips/boot/compressed/Makefile needs 64-bit.

I play with the AR7 platform, so VMLINUX_LOAD_ADDRESS is
0xffffffff94100000, and for an example 4MiB kernel
VMLINUZ_LOAD_ADDRESS is made out to be:
----
alex@berk:~$ bash -c 'printf "%x\n" $((0xffffffff94100000 + 0x400000))'
ffffffff94500000
alex@berk:~$ dash -c 'printf "%x\n" $((0xffffffff94100000 + 0x400000))'
80000000003fffff
----

The former is obviously correct whilst the later breaks things royally.

Fortunately working with only the lower 32bit's works for both bash and
dash:
----
$ bash -c 'printf "%x\n" $((0x94100000 + 0x400000))'
94500000
$ dash -c 'printf "%x\n" $((0x94100000 + 0x400000))'
94500000
----

So, we can split the original 64bit string to two parts, and only
calculate the low 32bit part, which is big enough (1GiB kernel sizes
anyone?) for a normal Linux kernel image file, now, we calculate the
VMLINUZ_LOAD_ADDRESS like this:

1. if present, append top 32bit of VMLINUX_LOAD_ADDRESS" as a prefix
2. get the sum of the low 32bit of VMLINUX_LOAD_ADDRESS + VMLINUX_SIZE

This patch fixes vmlinuz kernel builds on systems where only a
32bit-only math shell is available.

Patch Changelog:
  Version 2
    - simplified method by using 'expr' for 'substr' and making it work
	with dash once again
  Version 1
    - Revert the removals of '-n "$(VMLINUX_SIZE)"' to avoid the error
        of "make clean"
    - Consider more cases of the VMLINUX_LOAD_ADDRESS
  Version 0
    - initial release

Signed-off-by: Alexander Clouter <alex@digriz.org.uk>
Acked-by: Wu Zhangjin <wuzhangjin@gmail.com>
Patchwork: http://patchwork.linux-mips.org/patch/861/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>