k64: Rename security_frdm_k64f section

Renames the flash security section so it makes sense for other Kinetis
devices, not just k64. In Kinetis reference manuals, this section is
referred to as the 'flash configuration field'.

Change-Id: I2b7c7cc1ec2541419d77878d367d96c9ceb7a0cf
Signed-off-by: Maureen Helm <maureen.helm@nxp.com>