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>

lib/Makefile

@@ -5,6 +5,15 @@ endif
 ifdef CONFIG_NEWLIB_LIBC
 ZEPHYRINCLUDE += $(TOOLCHAIN_CFLAGS)
 ALL_LIBS += m c
+
+ifdef CONFIG_NEWLIB_LIBC_FLOAT_PRINTF
+LDFLAGS += -u _printf_float
+endif
+
+ifdef CONFIG_NEWLIB_LIBC_FLOAT_SCANF
+LDFLAGS += -u _scanf_float
+endif
+
 endif
 
 include $(srctree)/lib/iot/Makefile

misc/Kconfig

@@ -123,6 +123,21 @@ config NEWLIB_LIBC
 
 endchoice
 
+config NEWLIB_LIBC_FLOAT_PRINTF
+	bool "Build with newlib float printf"
+	default n
+	depends on NEWLIB_LIBC
+	help
+	Build with floating point printf enabled. This will increase the size of
+	the image.
+
+config NEWLIB_LIBC_FLOAT_SCANF
+	bool "Build with newlib float scanf"
+	default n
+	depends on NEWLIB_LIBC
+	help
+	Build with floating point scanf enabled. This will increase the size of
+	the image.
 
 config MINIMAL_LIBC_EXTENDED
 	bool "Build additional libc functions [EXPERIMENTAL]"