configs: tqma6: mba6: add and use defconfigs with general configurations

configs: tqma6: mba6: add and use defconfigs with general configurations

Add a common, MMC and SPI defconfigs with the general configurations
and use them in the variant-specific defconfigs.

while at it:

- set BOOTCOMMAND to mmcboot as it is used on majority of other TQ modules
  as the default
- remove DEFAULT_FDT_FILE - The kernel device tree is set at runtime.
- remove CONFIG_CMD_EXT4_WRITE - EXT4 with default features cannot be
  written from U-Boot.
- add CONFIG_FDT_FIXUP_PARTITIONS - This is needed to propagate MTD
  partition setup via devicetree to linux.

Signed-off-by: Paul Gerber <Paul.Gerber@ew.tq-group.com>
Signed-off-by: Markus Niebel <Markus.Niebel@ew.tq-group.com>
Signed-off-by: Max Merchel <Max.Merchel@ew.tq-group.com>