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
