KEYS: Use the symbol value for list size, updated by scripts/insert-sys-cert

When a certificate is inserted to the image using scripts/writekey, the
value of __cert_list_end does not change. The updated size can be found
out by reading the value pointed by the system_certificate_list_size
symbol.

Signed-off-by: Mehmet Kayaalp <mkayaalp@linux.vnet.ibm.com>
Signed-off-by: David Howells <dhowells@redhat.com>