board_types.txt: reserve board ID for MountainEagle H743 #1190

Job	Run time
build (base)	6m 19s
build (clang)	8m 16s
autotest (sitltest-rover)	16m 12s
autotest (sitltest-sailboat)	2m 37s
autotest (sitltest-balancebot)	7m 32s
	40m 56s

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