Enable warnings in EFI boot code

Set WARNS if not set for EFI boot code and fix the issues highlighted by
setting it.

Most components are set to WARNS level 6 with few being left at lower
levels due to the amount of changes needed to fix at higher levels.

Error types fixed:

• Missing / invalid casts
• Missing inner structs
• Unused vars
• Missing static for internal only funcs
• Missing prototypes
• Alignment changes
• Use of uninitialised vars
• Unknown pragma (intrinsic)
• Missing types etc due to missing includes
• printf formatting types

Reviewed by: emaste (in part)
MFC after: 2 weeks
X-MFC-With: r293268
Sponsored by: Multiplay
Differential Revision: https://reviews.freebsd.org/D4839