arm: u8500: Prefer building in thumb mode by default

Enabling CONFIG_SYS_THUMB_BUILD produces a significantly smaller
U-Boot binary (250 KiB vs 320 KiB) that still seems to be fully
functional. Make use of that by default but keep it as "imply" so it
can be disabled for testing in case this causes trouble for someone.

Signed-off-by: Stephan Gerhold <stephan@gerhold.net>