Add add & sub functions to template language

Add helper functions "add" and "sub" to the templating language to help
with basic math functions addition and subtraction with two arguments.

These functions can be used for some basic calculations in recipes, e.g:

   {{ add 1 1 }} => 2,
   {{ sub 1 1 }} => 0,
   {{ sub (add 1 1) 1 }} => 1,

Signed-off-by: Christopher Obbard <[email protected]>

actions/recipe.go

@@ -35,6 +35,8 @@ The following custom template functions are available
 - sector: Returns the argument * 512 (convential sector size) e.g. `{{ sector 64 }}`
 - escape: Shell escape the  argument `{{ escape $var }}`
 - decimal: Returns the integer argument as a decimal integer e.g. `{{ decimal 0x40 }} => 64`
+- add: Returns the calculation of two integer arguments `a + b`, e.g. `{{ add 1 1 }} => 2`
+- sub: Returns the calculation of two integer arguments `a - b`, e.g. `{{ sub 10 1 }} => 9`
 
 Mandatory properties for recipe:
 
@@ -161,6 +163,14 @@ func decimal(i int) int {
 	return i
 }
 
+func add(a int, b int) int {
+	return a + b
+}
+
+func sub(a int, b int) int {
+	return a - b
+}
+
 func DumpActionStruct(iface interface{}) string {
 	var a []string
 
@@ -249,6 +259,8 @@ func (r *Recipe) Parse(file string, printRecipe bool, dump bool, templateVars ..
 		"sector":  sector,
 		"escape":  escape,
 		"decimal": decimal,
+		"add":     add,
+		"sub":     sub,
 	}
 	t.Funcs(funcs)