cmake: hex: fix conversion of zero to hex

Previously, `to_hex(0 output)` would have placed `0x` into the `output`
variable, which is undesirable... this fix ensures that `0x0` is placed
into this variable if the input is zero.

Signed-off-by: Attie Grande <attie.grande@argentum-systems.co.uk>

cmake/hex.cmake

@@ -34,6 +34,10 @@ function(from_hex HEX DEC)
 endfunction()
 
 function(to_hex DEC HEX)
+    if(DEC EQUAL 0)
+        set(${HEX} "0x0" PARENT_SCOPE)
+        return()
+    endif()
     while(DEC GREATER 0)
         math(EXPR _val "${DEC} % 16")
         math(EXPR DEC "${DEC} / 16")