lib/randutils: Do not block on getrandom()

lib/randutils: Do not block on getrandom()

In Endless we have hit a problem when using 'sfdisk' on the really first
boot to automatically expand the rootfs partition. On this platform
'sfdisk' is blocking on getrandom() because not enough random bytes are
available. This is an ARM platform without a hwrng.

We fix this passing GRND_NONBLOCK to getrandom(). 'sfdisk' will use the
best entropy it has available and fallback only as necessary.

Signed-off-by: Carlo Caione <carlo@endlessm.com>