Allow svd2rust, form and cargo fmt to be replaced. Remove touch target.

Makefile

@@ -1,5 +1,7 @@
 DEVICES  := tm4c123x tm4c129x
 SVD2RUST ?= svd2rust
+FORM ?= form
+CARGO_FMT ?= cargo fmt
 
 .SECONDARY:
 
@@ -13,11 +15,7 @@ purge:
 	touch -t 198001010000 $(patsubst %,crates/%/src/lib.rs,$(DEVICES))
 
 .PHONY: rebuild
-rebuild: touch all
-
-.PHONY: touch
-touch:
-	touch ./targetdb/devices/*.xml
+rebuild: purge all
 
 svd/%-vendor.xml: data/%.xml overlay/%-interrupts.xml dslite2svd.rb
 	ruby dslite2svd.rb $(filter %.xml,$^) $@
@@ -39,4 +37,4 @@ svd/%.xml: overlay/%.patch svd/%-vendor.xml
 	fi
 
 crates/%/src/lib.rs: svd/%.xml
-	cd crates/$* && svd2rust -i ../../$< && rm -rf src && form -i lib.rs -o src/ && rm lib.rs && cargo fmt
+	cd crates/$* && $(SVD2RUST) -i ../../$< && rm -rf src && $(FORM) -i lib.rs -o src/ && rm lib.rs && $(CARGO_FMT)