machine/atsamd21: Add support for bootloader reset/programming and correct error in receiving endpoint 0 data for CDC Set Line Coding changes, implementing system reset on switch to 1200 baud connection speed with DTR false.

Signed-off-by: Ron Evans <ron@hybridgroup.com>

src/device/arm/arm.go

@@ -164,4 +164,8 @@ func SystemReset() {
 	// SCB->AIRCR  = ((0x5FA << SCB_AIRCR_VECTKEY_Pos)      |
 	//              SCB_AIRCR_SYSRESETREQ_Msk);
 	SCB.AIRCR.Set((0x5FA << SCB_AIRCR_VECTKEY_Pos) | SCB_AIRCR_SYSRESETREQ_Msk)
+
+	for {
+		Asm("wfi")
+	}
 }