system/skeleton: don't use random-seed from a read-only fs

A random-seed from a read-only filesystem is useless.

Also, drop the check for /etc/random-seed existence; it must exist after a
touch.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>