Stm32can: Port STM32L4 work to STM32

SAM34 Bugfixes

Removed  stm32f746g-disco/knsh configuration it failed to refresh (via tools/refresh.sh).  I assume that it is a hand-edited configuration and, hence, must be removed from the repository

sdmmc1 for stm32F7 74-75

Add some CAN mode IOCTL calls.  These will be useful for device autotest when the application boots.  They are redundant with the CONFIG_CAN_LOOPBACK option, which can now just be interpreted as a default setting.

syslog_dev_flush() needs to check if the inode is a mountpoint before calling the flush() method.  Noted by David Sidrane.

can: Add support for poll