serial# is one of the vendor properties and thus protected from being
overwritten if already set. If env_set is called anyway this result in
some nasty warnings, so check for presence before trying that.

In the same direction check for the presence of cpuid# and compare it
to the actual hardware and emit a warning if they don't match.

Signed-off-by: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>