Add readkey and fix putchar for more targets.

include/a8_kernel.mfk

@@ -3,7 +3,7 @@
 #warn a8_kernel module should be used only on Atari computer-compatible targets
 #endif
 
-asm void putchar(byte a) {
+noinline asm void putchar(byte a) {
     ? tax
     lda $347
     pha
@@ -13,6 +13,16 @@ asm void putchar(byte a) {
     rts
 }
 
+noinline asm byte readkey() {
+    lda #$C
+    sta $2A
+    lda $E425
+    pha
+    lda $E424
+    pha
+    rts
+}
+
 inline void new_line() {
     putchar($9b)
 }