canserial: Rename canbus.c to canserial.c

Rename the canbus.c code to canserial.c and introduce new wrapper
functions in canbus.c that connect the low-level canbus hardware code
to the high-level canserial.c code.

This is in preparation for adding "usb to canbus bridge mode".

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

src/stm32/can.c

@@ -12,6 +12,7 @@
 #include "command.h" // DECL_CONSTANT_STR
 #include "generic/armcm_boot.h" // armcm_enable_irq
 #include "generic/canbus.h" // canbus_notify_tx
+#include "generic/canserial.h" // CANBUS_ID_ADMIN
 #include "internal.h" // enable_pclock
 #include "sched.h" // DECL_INIT