bmpblk: Centralize functions in Converter class

Move functions converting texts to PNGs into Converter class, and
merge with their counterpart which converts PNGs to BMPs. Move path
constants from global variables to Converter class attributes.

Also move load_boards_config() and check_fonts() near the top of file.

BUG=b:178458718
TEST=make BOARD=asurada
TEST=Generated bitmaps are the same as before
BRANCH=none

Change-Id: I3a25a6728b2a35fb68c88aeb37332e197aa42e9d
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/bmpblk/+/2658368
Tested-by: Yu-Ping Wu <[email protected]>
Commit-Queue: Yu-Ping Wu <[email protected]>
Reviewed-by: Joel Kitching <[email protected]>
1 file changed