samples/mbedtls_dtls_client: Fix wild write in entropy_source

The example entropy_source implementation should write entropy to the
output buffer rather than to the context pointer which in this example
happens to be NULL.

Take the opportunity to reorganize the entropy_source to use all of
the entropy provided by a call to sys_rand32_get() rather than just
1/4 of it.

The entropy_source() callback from mbedtls is given a maximum amount
of entropy to return, rather than a minimum amount.  Hence it makes
more sense to deliver exactly one chunk (32 bits) of entropy from the
call to sys_rand32_get() per call and let the mbedtls entropy handler
worry about how much entropy we actually need to collect (ie the threshold).

Change-Id: I57ed438de5cb1223619fde0fb8039d6eca284646
Signed-off-by: Marcus Shawcroft <marcus.shawcroft@arm.com>