stand: Make BIOS loader size limits settable

It's sometimes desirable to override the size limit: It's a soft limit
and there are times we exceed the limit by just a little bit and don't
want the build to fail (or we are hitting runtime failures below the
510,000 byte limit).

Sponsored by: Netflix

(cherry picked from commit 7d72ff905770a64a22125c890438189db71104ae)