bvb_refimpl: Add script to check symbols.

This change introduces a script to verify only symbols starting with
bvb_ are used in the reference implementation. This is to catch mistakes
where the standard C library (e.g. malloc() or printf()) is used by

TEST=Manually tested by using malloc() in bvb_verify().

Change-Id: I54bc1bf30d7081cf60c49720b5f8d82e563ffa21
3 files changed