pkg/jerryscript: namespace BUILD_DIR variable

BUILD_DIR was introduced but the variable was already used by
`pkg/jerryscript` so there is a name collision.
Namespace it to prevent issues.

pkg/jerryscript/Makefile.jerryscript

@@ -1,4 +1,4 @@
-BUILD_DIR  ?= $(BINDIR)/jerryscript
+JERRYSCRIPT_BUILD_DIR  ?= $(BINDIR)/jerryscript
 
 JERRYHEAP  ?= 16
 
@@ -27,7 +27,7 @@ endif
 all: libjerry
 
 libjerry:
-	cmake -B$(BUILD_DIR) -H./ \
+	cmake -B$(JERRYSCRIPT_BUILD_DIR) -H./ \
 	 -DCMAKE_SYSTEM_NAME=RIOT \
 	 -DCMAKE_SYSTEM_PROCESSOR="$(MCPU)" \
 	 -DCMAKE_C_COMPILER=$(CC) \
@@ -42,7 +42,7 @@ libjerry:
 	 -DEXTERNAL_COMPILE_FLAGS="$(INCLUDES) $(EXT_CFLAGS)" \
 	 -DMEM_HEAP_SIZE_KB=$(JERRYHEAP)
 
-	"$(MAKE)" -C $(BUILD_DIR) jerry-core jerry-ext jerry-port-default-minimal
-	cp $(BUILD_DIR)/lib/libjerry-core.a $(BINDIR)/jerryscript.a
-	cp $(BUILD_DIR)/lib/libjerry-ext.a $(BINDIR)/jerryscript-ext.a
-	cp $(BUILD_DIR)/lib/libjerry-port-default-minimal.a $(BINDIR)/jerryport-minimal.a
+	"$(MAKE)" -C $(JERRYSCRIPT_BUILD_DIR) jerry-core jerry-ext jerry-port-default-minimal
+	cp $(JERRYSCRIPT_BUILD_DIR)/lib/libjerry-core.a $(BINDIR)/jerryscript.a
+	cp $(JERRYSCRIPT_BUILD_DIR)/lib/libjerry-ext.a $(BINDIR)/jerryscript-ext.a
+	cp $(JERRYSCRIPT_BUILD_DIR)/lib/libjerry-port-default-minimal.a $(BINDIR)/jerryport-minimal.a