README: mxc_hab: Adapt the CONFIG_SECURE_BOOT text to Kconfig

Commit 6e1f4d26 ("arm: imx-common: add SECURE_BOOT option to
Kconfig") moved the CONFIG_SECURE_BOOT option to Kconfig, so update
the mxc_hab README file to reflect that.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Gary Bisson <gary.bisson@boundarydevices.com>