keyspan_pda: Add makefile for building from source

The commands given in the source aren't quite right, and don't cover
use of ihex2fw.
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

WHENCE

@@ -173,6 +173,8 @@ Source: keyspan_pda/keyspan_pda.S
 File: keyspan_pda/xircom_pgs.fw
 Source: keyspan_pda/xircom_pgs.S
 
+Source: keyspan_pda/Makefile
+
 Licence: GPLv2+
 
 Compiled from original 8051 source into Intel HEX, used in our binary ihex form.

keyspan_pda/Makefile

+CC = gcc
+IHEX2FW = ihex2fw
+
+all: keyspan_pda.fw xircom_pgs.fw
+
+%.fw: %.S
+	$(CC) -x assembler-with-cpp -P -E -o $*.asm $<
+	as31 -l $*.asm
+	$(IHEX2FW) $*.hex $@
+
+clean:
+	rm -f *.asm *.fw *.hex *.lst *.obj