Merged in david_s5/nuttx-2/david_s5/stm32_otgfsdevc-edited-online-with-bitbu-1472231999710 (pull request #126)

stm32_otgfsdev.c edited online with Bitbucket

dup SOF removed as noted by Sébastien Lorquet

BugFix:Lost first word from FIFO

    1) Do not overwrite Reserved Bits in GINTSTS (per ref manual)*
    2) Acknowledge all pending int on entry to ISR that are Only rc_w1*
    3) Do not disable RXFVL*
    4) Loop until RXFVL is cleared*
    5) Only clear the NAK on the endpoint on the OTGFS_GRXSTSD_PKTSTS_SETUPDONE to
       not loose the first WORD of FIFO all the data  (Bug Fix)

Changed marked *are just driver clean up and ensure ints are not lost.
The bug fix is #5

Test case open putty and observer the Set/Get LineCoding
Without this fix #5 the Get will not match the Set, and
infact the data might be skewed by 4 bytes, that are lost
from the FIFO if the OTGFS_DOEPCTL0_CNAK bit is set in the
OTGFS_GRXSTSD_PKTSTS_SETUPRECVD as opposed to the OTGFS_GRXSTSD_PKTSTS_SETUPDONE

Set Line Coding DATA1: 4B | 00 c2 01 00 00 00 08 | c8 1B
Get Line Coding DATA1: 4B | .. .. .. .. 00 00 08   c8 .. 00 00 07 | 7a 72

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.