pkg/jerryscript: build into '$(BINDIR)'

Build into the BINDIR directory instead of the source repository.
This makes 'clean' work as expected without other intervention.

Also it goes in the direction of having the package source repository
board independent.

pkg/jerryscript/Makefile.jerryscript

-BUILD_DIR  ?= $(CURDIR)/riot
+BUILD_DIR  ?= $(BINDIR)/jerryscript
 
 JERRYHEAP  ?= 16