Makefile: Add support for INSTALL_PREFIX

Add basic support for INSTALL_PREFIX

make install INSTALL_PREFIX=/usr/local
make uninstall INSTALL_PREFIX=/usr/local

See: raspberrypi#208

Makefile

@@ -1,5 +1,6 @@
 PKG_VER=$(shell grep rpiboot debian/changelog | head -n1 | sed 's/.*(\(.*\)).*/\1/g')
 GIT_VER=$(shell git rev-parse HEAD 2>/dev/null | cut -c1-8 || echo "")
+INSTALL_PREFIX?=/usr
 
 rpiboot: main.c bootfiles.c msd/bootcode.h msd/start.h msd/bootcode4.h msd/start4.h
 	$(CC) -Wall -Wextra -g -o $@ main.c bootfiles.c `pkg-config --cflags --libs libusb-1.0` -DGIT_VER="\"$(GIT_VER)\"" -DPKG_VER="\"$(PKG_VER)\""
@@ -14,17 +15,17 @@ bin2c: bin2c.c
 	$(CC) -Wall -Wextra -g -o $@ $<
 
 install: rpiboot
-	install -m 755 rpiboot /usr/bin/
-	install -d /usr/share/rpiboot
-	install -d /usr/share/rpiboot/msd
-	install -m 644 msd/bootcode.bin  /usr/share/rpiboot/msd
-	install -m 644 msd/bootcode4.bin /usr/share/rpiboot/msd
-	install -m 644 msd/start.elf  /usr/share/rpiboot/msd
-	install -m 644 msd/start4.elf /usr/share/rpiboot/msd
+	install -m 755 rpiboot $(INSTALL_PREFIX)/bin/
+	install -d $(INSTALL_PREFIX)/share/rpiboot
+	install -d $(INSTALL_PREFIX)/share/rpiboot/msd
+	install -m 644 msd/bootcode.bin  $(INSTALL_PREFIX)/share/rpiboot/msd
+	install -m 644 msd/bootcode4.bin $(INSTALL_PREFIX)/share/rpiboot/msd
+	install -m 644 msd/start.elf  $(INSTALL_PREFIX)/share/rpiboot/msd
+	install -m 644 msd/start4.elf $(INSTALL_PREFIX)/share/rpiboot/msd
 
 uninstall:
-	rm -f /usr/bin/rpiboot
-	rm -rf /usr/share/rpiboot
+	rm -f $(INSTALL_PREFIX)/bin/rpiboot
+	rm -rf $(INSTALL_PREFIX)/share/rpiboot
 
 clean:
 	rm -f rpiboot msd/*.h bin2c