Add preliminary bootloader

This bootloader right now just prints the letter A, but should be
capable of reading blocks from the SD card into memory.

sw/bootloader/bios_calls.s

@@ -0,0 +1,23 @@
+.export _sd_readblock
+
+.autoimport
+
+.importzp sreg
+
+; sreg is the pointer to store the data
+; a/y is the block address
+; send command 17 with the block address of 00/y/a
+
+; void sd_readblock(uint16_t addr, void* buf);
+
+_sd_readblock:
+    sta sreg        ; move buf pointer to sreg
+    stx sreg+1
+
+    jsr popax       ; move addr to a/y
+    tya
+
+    ldx #$1         ; call interrupt 1
+    brk
+
+    rts