usr.sbin/flashctl: skip lint's strict bool mode with Clang

The strict bool mode is already checked in GCC mode, so restore the
previous idiomatic code.

flashctl: enable lint's strict bool mode

No binary change.

Add manpage for flashctl

Import the Flash and NAND subsytem code contributed by the University
of Szeged, Hungary.

The commit includes:
- Flash layer, which gives a common API to access flash devices
- NAND controller sub

Import the Flash and NAND subsytem code contributed by the University
of Szeged, Hungary.

The commit includes:
- Flash layer, which gives a common API to access flash devices
- NAND controller subsystem for the flash layer
- An example OMAP driver which is used on BeagleBoard or alike ARM boards

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