newlib: add support for nano-formatted-io

If newlib is configured with --enable-newlib-nano-formatted-io,
floating-point support is split out of the formatted I/O code into
weak functions which are not linked by default. This leads to a smaller
code by about 16~20k when using newlib "printf" and/or "sscanf" but not
using floating point I/O.

Programs that need floating-point I/O support must explicitly request
linking of one or both of the floating-point functions:

    _printf_float or _scanf_float.

This can be done at link time using the -u option which can be passed
to either gcc or ld.

Implemented via new configuration options:
    CONFIG_NEWLIB_LIBC_FLOAT_PRINTF
    CONFIG_NEWLIB_LIBC_FLOAT_SCANF

Change-Id: I57f9d9f02e6d21d6011d14de7153b1d3ba6f6e32
Signed-off-by: Juro Bystricky <juro.bystricky@intel.com>