Add PYTHON definition to Makefile.

Add PYTHON definition to Makefile so users can override it.  This
allows users to specify an exact python executable name to use during
the build.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
diff --git a/Makefile b/Makefile
index 0343ce5..513e8f1 100644
--- a/Makefile
+++ b/Makefile
@@ -63,6 +63,7 @@
 OBJCOPY=objcopy
 OBJDUMP=objdump
 STRIP=strip
+PYTHON=python
 
 # Default targets
 -include $(KCONFIG_CONFIG)
@@ -152,7 +153,7 @@
 	$(Q)$(OBJDUMP) -thr $(OUT)code32flat.o > $(OUT)code32flat.o.objdump
 	$(Q)$(OBJDUMP) -thr $(OUT)code32seg.o > $(OUT)code32seg.o.objdump
 	$(Q)$(OBJDUMP) -thr $(OUT)code16.o > $(OUT)code16.o.objdump
-	$(Q)./tools/layoutrom.py $(OUT)code16.o.objdump $(OUT)code32seg.o.objdump $(OUT)code32flat.o.objdump $(OUT)romlayout16.lds $(OUT)romlayout32seg.lds $(OUT)romlayout32flat.lds
+	$(Q)$(PYTHON) ./tools/layoutrom.py $(OUT)code16.o.objdump $(OUT)code32seg.o.objdump $(OUT)code32flat.o.objdump $(OUT)romlayout16.lds $(OUT)romlayout32seg.lds $(OUT)romlayout32flat.lds
 
 # These are actually built by tools/layoutrom.py above, but by pulling them
 # into an extra rule we prevent make -j from spawning layoutrom.py 4 times.
@@ -174,7 +175,7 @@
 	@echo "  Prepping $@"
 	$(Q)$(OBJDUMP) -thr $< > $<.objdump
 	$(Q)$(OBJCOPY) -O binary $< $(OUT)bios.bin.raw
-	$(Q)./tools/checkrom.py $<.objdump $(OUT)bios.bin.raw $(OUT)bios.bin
+	$(Q)$(PYTHON) ./tools/checkrom.py $<.objdump $(OUT)bios.bin.raw $(OUT)bios.bin
 	$(Q)$(STRIP) -R .comment $< -o $(OUT)bios.bin.elf
 
 
@@ -204,15 +205,15 @@
 
 $(OUT)vgabios.bin: $(OUT)vgabios.bin.raw tools/buildrom.py
 	@echo "  Finalizing rom $@"
-	$(Q)./tools/buildrom.py $< $@
+	$(Q)$(PYTHON) ./tools/buildrom.py $< $@
 
 ####### dsdt build rules
 src/%.hex: src/%.dsl ./tools/acpi_extract_preprocess.py ./tools/acpi_extract.py
 	@echo "Compiling DSDT"
 	$(Q)cpp -P $< > $(OUT)$*.dsl.i.orig
-	$(Q)./tools/acpi_extract_preprocess.py $(OUT)$*.dsl.i.orig > $(OUT)$*.dsl.i
+	$(Q)$(PYTHON) ./tools/acpi_extract_preprocess.py $(OUT)$*.dsl.i.orig > $(OUT)$*.dsl.i
 	$(Q)iasl -l -tc -p $(OUT)$* $(OUT)$*.dsl.i
-	$(Q)./tools/acpi_extract.py $(OUT)$*.lst > $(OUT)$*.off
+	$(Q)$(PYTHON) ./tools/acpi_extract.py $(OUT)$*.lst > $(OUT)$*.off
 	$(Q)cat $(OUT)$*.off > $@
 
 $(OUT)ccode32flat.o: src/acpi-dsdt.hex src/ssdt-proc.hex src/ssdt-pcihp.hex