board: tbs2910: disable loadb and loads commands

The loadb and loads commands are not needed for booting.

There are also more reliable and faster alternatives to
loadb and loads that can be used with the current configuration.

As that the resulting image is already very close to the
size limit, removing the loadb and loads commands shouldn't
hurt.

With arm-linux-gnueabi-gcc 9.2.0-1 from the Parabola
GNU/Linux distribution we have the following size
reduction:
- text: 6733 bytes
- data: 116 bytes
- bss: 1172 bytes
- total: 8021 bytes
Signed-off-by: NDenis 'GNUtoo' Carikli <GNUtoo@cyberdimension.org>
Acked-by: NSoeren Moch <smoch@web.de>

configs/tbs2910_defconfig

@@ -28,6 +28,8 @@ CONFIG_SYS_MEMTEST_START=0x10000000
 CONFIG_SYS_MEMTEST_END=0x2f400000
 CONFIG_CMD_GPIO=y
 CONFIG_CMD_I2C=y
+# CONFIG_CMD_LOADB is not set
+# CONFIG_CMD_LOADS is not set
 CONFIG_CMD_MMC=y
 CONFIG_CMD_PART=y
 CONFIG_CMD_PCI=y