Codec2: generate documentation via make

Usage: mmm codec2:DOC_TARGET

DOC_TARGETs:

    docs-api: API documentation
    requires doxygen
    output: $(OUT_DIR)/target/common/docs/codec2/api/index.html

    docs-internal: all Codec2 packages documentation (including tests
        and platform support)
    requires doxygen
    output: $(OUT_DIR)/target/common/docs/codec2/internal/index.html

    docs: build all doc targets

Change-Id: Iae7540ca4d0cc011e41760f1549bacd837189140