That's just a bit of cmd line building. Three clauses to build a string -

  If BOARDNAME is defined, add board=$(BOARDNAME) to the string
  If MTDPARTS is defined, add mtdparts=$(MTDPARTS) to the string
  If CONSOLE is defined, add console=$(CONSOLE) to the string

Pretty simple, if a little like the ternary operator in C.

You must have seen more complex clauses than that in shell scripts and all sorts of places.