Code refactor and bug fixes related to bgm. #2063

Job	Run time
build-variants (EXTRA_FEATURES=0, PADEMU=0)	1m 8s
build-variants (EXTRA_FEATURES=0, PADEMU=1)	1m 8s
build	1m 2s
build-lang	31s
build-variants (EXTRA_FEATURES=1, PADEMU=0)	1m 16s
build-debug (iopcore_debug, :main)	1m 17s
build-variants (EXTRA_FEATURES=1, PADEMU=1)	1m 11s
build-debug (ingame_debug, :main)	1m 16s
build-debug (eesio_debug, :main)	1m 18s
build-debug (iopcore_ppctty_debug, :main)	1m 15s
build-debug (ingame_ppctty_debug, :main)	1m 16s
build-debug (DTL_T10000=1, :main)	1m 17s
merge-variants	4s
release	0s
merge-debug	8s
	14m 7s

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