rtos: Support rt-kernel

Works for the proprietary rt-kernel from rt-labs.

See: https://rt-labs.com/product/rt-kernel/
Change-Id: Id2c2e292c15fb17eab25e3d07db05014daa2a2b0
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6668
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>

src/rtos/Makefile.am

@@ -21,6 +21,7 @@ noinst_LTLIBRARIES += %D%/librtos.la
 	%D%/mqx.c \
 	%D%/uCOS-III.c \
 	%D%/nuttx.c \
+	%D%/rtkernel.c \
 	%D%/hwthread.c \
 	%D%/zephyr.c \
 	%D%/riot.c \