Remove BootFlash borrow

Compiler will infer a different lifetime for BootFlash than for the
borrowed flash, which makes it require more type annotations than if it
was just owning the type. Since it doesn't really matter if it owns or
borrows in practical use, change it to own so that it simplifies usage.

examples/boot/bootloader/nrf/src/main.rs

@@ -21,7 +21,7 @@ fn main() -> ! {
 
     let mut bl = BootLoader::default();
     let start = bl.prepare(&mut SingleFlashConfig::new(&mut BootFlash::<_, 4096>::new(
-        &mut WatchdogFlash::start(Nvmc::new(p.NVMC), p.WDT, 5),
+        WatchdogFlash::start(Nvmc::new(p.NVMC), p.WDT, 5),
     )));
     unsafe { bl.load(start) }
 }