libc/newlib: Add CUSTOM version and CUSTOM_LOCATION config options and GetCustom

CUSTOM_LOCATION config options only presented in menuconfig if component
CUSTOM version selected.

Signed-off-by: "David Holsgrove" <david.holsgrove@xilinx.com>
[yann.morin.1998@free.fr: fix indentation]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Message-Id: <d02252752d4dc5e68ae3.1349931202@localhost.localdomain>
PatchWork-Id: 190795

 # Bare metal config options

 ## select BARE_METAL

 ##

 ## help 'Bare metal' refers to those programs that run without any kernel.

 ## help

 ## help You probably want to say 'y' here if you plan to use your compiler

 ## help to build bootloaders. It is not yet suitable to build Linux kernels,

 ## help though, because the APCI stuff relies on the target C library headers

 ## help being available?!?!...