doc: eficonfig: add description for UEFI Secure Boot Configuration

This commits adds the description for the UEFI Secure Boot
Configuration through the eficonfig menu.

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>

Redacted the complete document.
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>