Updata difftime.  Add a version of difftime for the case where the platform does not support type double

libc/time: This commit adds the difftime() function.  Since the function returns a double, I have isolated it in a CONFIG_LIBC_DIFFTIME option (It also depends on the toolchain-dependent CONFIG_HAVE_DOUBLE so is not available on tiny platforms).

Update to NuttX C coding style document with additions discussing long comments on the right side of a statement or data definition.

Merged in bkueng/nuttx-1/bkueng/mtd-add-fujistu-mb85rs256b-ramtron-suppo-1471976814505 (pull request #124)

mtd: Add Fujistu MB85RS256B ramtron support

contributed by flatlevel from https://github.com/PX4/NuttX/pull/79

CXXFLAGS: add -fcheck-new whenever -fno-exceptions is used

This is to avoid GCC to optimize null-pointer checks away, in case the
default operator new is used together with -fno-exceptions.

XorShift128:  Add some protection to handle the case where this logic could be called from the interrupt level.

sched/sched_cpuload_oneshot:  Use the oneshot timer with optional entropy to measuer cPU load if so configured.

Add missing eth support in stm32butterfly2/nshnet config

Fix compilation warnings for stm32 eth with certain configs

Fix wrong config for stm32butterfly2/nshusbhost