video: bmp: rename CONFIG_BMP_24BMP to CONFIG_BMP_24BPP

Due to a typo, the 24 bit-per-pixel configuration ends in 24BMP
instead of 24BPP. This change renames it throughout the source tree
for consistency and to make moving these options into Kconfig easier
and less error-prone.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Hannes Schmelzer <hannes.schmelzer@br-automation.com>