cortexm: detach still needs extra cycles.

src/target/cortexm.c

@@ -414,6 +414,8 @@ void cortexm_detach(target *t)
 
 	/* Disable debug */
 	target_mem_write32(t, CORTEXM_DHCSR, CORTEXM_DHCSR_DBGKEY);
+	/* Add some clock cycles to get the CPU running again.*/
+	target_mem_read32(t, 0);
 }
 
 enum { DB_DHCSR, DB_DCRSR, DB_DCRDR, DB_DEMCR };