small startup/interrupt cleanup

c64/kernal/interrupts/hw_entry_nmi.s

@@ -0,0 +1,20 @@
+
+hw_entry_nmi:
+
+	sei // don't allow IRQ to interfere, see https://www.c64-wiki.com/wiki/Interrupt
+
+#if CONFIG_CPU_MOS_6502 && CONFIG_BCD_SAFE_INTERRUPTS
+	cld // clear decimal flag to allow using it without disabling interrupts
+#endif
+
+	// Call interrupt routine (only if initialised)
+	pha
+	lda NMINV
+	ora NMINV+1
+	beq !+
+	pla
+	jmp (NMINV)
+!:
+	// Vector not initialized - call default interrupt routine
+	pla
+	jmp default_nmi_handler