Only look for boostrap useful options:
- PACKAGESITE
- ABI
- MIRROR_TYPE
- ASSUME_ALWAYS_YES
While here makes PACKAGESITE expand the ${ABI} variable.
Allow to deactivate any SRV record look up (MIRROR_TYPE=none)
Use the same mechanism as for pkgng itself: first get configuration out of
environment variable and fallback on pkg.conf if exists.
Reviewed by: bdrewery
