tbs2910: Disable Plan9/RTEMS bootm support

We have once again reached a point where this board does not build in
some cases with supported toolchains due to reaching a size constraint.
To regain some space, disable support for Plan 9 / RTEMS images with the
bootm command.
Acked-by: NStefano Babic <sbabic@denx.de>
Signed-off-by: NTom Rini <trini@konsulko.com>

configs/tbs2910_defconfig

@@ -17,6 +17,8 @@ CONFIG_BOARD_EARLY_INIT_F=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="Matrix U-Boot> "
 CONFIG_CMD_BOOTZ=y
+# CONFIG_BOOTM_PLAN9 is not set
+# CONFIG_BOOTM_RTEMS is not set
 # CONFIG_CMD_FDT is not set
 CONFIG_CMD_MEMTEST=y
 # CONFIG_CMD_FLASH is not set