mel: don't use BB_HASHBASE_WHITELIST_append

Currently, bitbake isn't always using the finalized configuration metadata to
get the value of this variable, so the _append isn't always applied.

JIRA: MEIBPADIT-811

Signed-off-by: Christopher Larson <kergoth@gmail.com>
Merge pull request #227 from kergoth/MEIBPADIT-811

mel: don't use BB_HASHBASE_WHITELIST_append