Support simple variable assignment

Rakefile

@@ -44,13 +44,15 @@ task :convert do
   end
 end
 
+rule /^build\/[[:alpha:]]+\/stage[[:digit:]]+$/ => ->(match) {
+  "#{match}.s"
+} do |t|
+  sh CROSS_GCC, '-nostdlib', '-o', t.name, *t.prerequisites
+end
+
 STAGES.each do |stage|
   previous = stage.delete_prefix('stage').to_i.pred
 
-  file "build/valid/#{stage}" => "build/valid/#{stage}.s" do |t|
-    sh CROSS_GCC, '-nostdlib', '-o', t.name, *t.prerequisites
-  end
-
   file "build/valid/#{stage}.s" => ["build/boot/#{stage}", "boot/#{stage}.elna"] do |t|
     exe, source = t.prerequisites
 
@@ -61,10 +63,6 @@ STAGES.each do |stage|
     IO.copy_stream last_stdout, t.name
   end
 
-  file "build/boot/#{stage}" => "build/boot/#{stage}.s" do |t|
-    sh CROSS_GCC, '-nostdlib', '-o', t.name, *t.prerequisites
-  end
-
   file "build/boot/#{stage}.s" => ["build/valid/stage#{previous}", "boot/#{stage}.elna"] do |t|
     exe, source = t.prerequisites
 
@@ -80,12 +78,6 @@ end
 # Stage 1.
 #
 
-file 'build/valid/stage1' => ['build/valid', 'build/valid/stage1.s'] do |t|
-  source = t.prerequisites.select { |prerequisite| prerequisite.end_with? '.s' }
-
-  sh CROSS_GCC, '-nostdlib', '-o', t.name, *source
-end
-
 file 'build/valid/stage1.s' => ['build/boot/stage1', 'boot/stage1.s', 'build/valid'] do |t|
   source, exe, = t.prerequisites.partition { |prerequisite| prerequisite.end_with? '.s' }