configs: legoev3: adjust to reduce binary size

configs: legoev3: adjust to reduce binary size

Adjust legoev3_defconfig to reduce the binary output size.

As u-boot has bloated a bit over the years, the legoev3_defconfig can no
longer build something that fits in the 256kB size limit of the EV3.
This drops a few unused features, but the real difference-makers are
enabling thumb instructions and using link time optimization to reduce
the size.

This reduced u-boot.bin from 279,920 to 198,416 bytes on my local
machine with arm-none-eabi-gcc (15:13.2.rel1-2) 13.2.1 20231009.

HAS_BOARD_SIZE_LIMIT is also added to catch any future regressions.

Signed-off-by: David Lechner <david@lechnology.com>