Enforce DEFAULTTUNE across all platforms/architectures/BSPs

AGL should as 'distro' enforce a common set of compiler tune flags instead of letting each BSP choose its own. This will reduce the number of BSP's needed to compile applications down to:

• i586 (needed?)
• x86_64
• arm32 (e.g. vfpv4/neon + thumb2)
• arm64 (e.g. ??)

No BSP should override the DISTRO settings (?= or ??=).

Outcome:

• just one BSP needed for arm32 , arm64, x86 .