pkg/jerryscript: namespace BUILD_DIR variable

[cladmi]

Contribution description

I forgot to git grep when merging BUILD_DIR and I found issues about it.

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

Testing procedure

Build examples/javascript and the build directory should not be created, and the archives will be generated in examples/javascript/bin/native/jerryscript/lib/
When building in docker, build will be created but empty.

There is no more other usages now in the repository:

git grep '(BUILD_DIR)'
Makefile.include:override BUILD_DIR      := $(abspath $(BUILD_DIR))
makefiles/docker.inc.mk:        $(Q)mkdir -p $(BUILD_DIR)
makefiles/docker.inc.mk:            -v '$(BUILD_DIR):$(DOCKER_BUILD_ROOT)/build' \

Issues/PRs references

Original PR #10038

[miri64]

ACK straight forward change and jerryscript still compiles.

[cladmi]

Thanks for the review.