Try with search_path for some avb path args

If we can't find relative signing_helper path, try with the OPTIONS.search_path dir prefix， and integrate the avb_*_key_path for the same logic.

Test: Build && releasetools_test

Signed-off-by: zhangyongpeng <zhangyongpeng@xiaomi.com>
Change-Id: Ifb1096ddea90693668f3344eb242bf9725113d11