board: tbs2910_defconfig: Add support for booting general purpose distributions

board: tbs2910_defconfig: Add support for booting general purpose distributions

This is needed to enable distro_boot support later on.

Adding the necessary configuration to enable booting general
purpose distributions is typically done by enabling
CONFIG_DISTRO_DEFAULTS.

However the resulting image size is is already very close to
the size limit and PXE isn't strictly needed, so instead we
made sure that all the configuration selected by
CONFIG_DISTRO_DEFAULTS were selected, with the exception of
PXE related configuration (CONFIG_BOOTP_PXE and
CONFIG_CMD_PXE) that are both disabled.

With arm-linux-gnueabi-gcc 9.3.0-1 from the Parabola
GNU/Linux distribution, we have the following size
differences:
- text: +7526 bytes
- data: +28 bytes
- bss: -12 bytes
- total: +7542 bytes

Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo@cyberdimension.org>
Reviewed-by: Soeren Moch <smoch@web.de>