lvgl: Flush thread can have preemptive priority

The LVGL flush thread was hard-coded to be cooperative.
For long-running actions like data transfer to the display,
this is problematic as it might block high-frequency actions like
USB or input events.

Hence, make it configurable to be preemptive, and rename it to match the
similar Kconfig values like CONFIG_SDL_THREAD_PRIORITY.

Signed-off-by: Martin Stumpf <finomnis@gmail.com>