MIPS: ZBOOT: gather string functions into string.c

In the worst case this adds less then 128 bytes of code
but on the other hand this makes code organization more clear.

Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: John Crispin <blogic@openwrt.org>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/6344/