tbs2910: Disable VxWorks image booting support

There are currently no known users of this functionality on this
platform, disable it to prepare for additional VxWorks functionality
that would cause this platform to fail to link.

Cc: Soeren Moch <smoch@web.de>
Signed-off-by: NTom Rini <trini@konsulko.com>
Acked-by: NSoeren Moch <smoch@web.de>

configs/tbs2910_defconfig

@@ -21,6 +21,7 @@ CONFIG_SYS_PROMPT="Matrix U-Boot> "
 CONFIG_CMD_BOOTZ=y
 # CONFIG_BOOTM_PLAN9 is not set
 # CONFIG_BOOTM_RTEMS is not set
+# CONFIG_BOOTM_VXWORKS is not set
 # CONFIG_CMD_FDT is not set
 CONFIG_CMD_MEMTEST=y
 # CONFIG_CMD_FLASH is not set