can: Add support for poll

SYSLOG: Change configuration selections to assure that one and only one SYSLOG device can be selected.  Also add a check to assure that it is appropriate to use up_putc as the low-level syslog device.  Adds CONFIG_SYSLOG_CONSOLE and CONFIG_SYSLOG_SERIAL_CONSOLE.

SYSLOG: Now a two phase initialization. Some SYSLOG channels cannot be initialized until later in the bringup

-remove a wrong comment in atmel mcan ioctl
-add ioctls to set/get bit timing in stm32l4
-add ioctl hooks to allow future management of can id filters.

assert: Don't define static_assert in C++

Replaces last three commits.  Does the same thing, but does it in a way that does not change the usage model.

Break syslog_channel.c up into syslog_channel.c, syslog_putc.c, syslog_force.c and syslog_flush.c to limited what is brought into the link.  Separate syslog_emergstream.c from syslog_stream.c.  Didn't help in the case I was looking at.

Make system: Build the drivers/ directory even if file descriptors are disabled.  There are still things that may be needed from the drivers/ director (such as SYSLOG logic or lower half drivers).

Remove some last traces of lowvsyslog that were missed; Add a SYSLOG emergency channel for handling assertion output more cleanly

assert: Define static_assert

Remove lowsyslog().  The new syslog() includes all of the functionality of lowsyslog().  No longer any need for two interfaces.

Since syslog_add_intbuffer can now be called from the IDLE thread, it must use a critical section.  Also fix an error in circular buffer index handling.