Krakout: Wait until no key is pressed

BCAD: Wait until no key is pressed

Loops until all keys are released.
wait_no_key	BCAD	LD BC,$00FE	Select all keyboard half-rows at once.
	BCB0	IN A,(C)	Read them.
	BCB2	AND $1F	Keep the five key bits.
	BCB4	XOR $1F	Invert the value read, then loop while any key is still down.
	BCB6	JR NZ,wait_no_key
	BCB8	RET

Routines