core: fix arithmetic on a pointer to void

Use char* instead of void* for this
pointer arithmetic. Fixes the following:

error: arithmetic on a pointer to void
buf = buf + offset;
      ~~~ ^

Change-Id: I6cfc92c9240fda66a52e780cda7820fdba76914a
Merge "core: fix arithmetic on a pointer to void"
Remove direct reference to X509CertImpl

This should use the system default X.509 certificate provider.

Change-Id: Ibdfd800a9d10f9dc566a360401efe7c3dc6cef5d
Merge "Remove direct reference to X509CertImpl"