efi_loader: size of secure boot variables

The variables SetupMode, AuditMode, DeployedMode are explicitly defined as
UINT8 in the UEFI specification. The type of SecureBoot is UINT8 in EDK2.

Use variable name secure_boot instead of sec_boot for the value of the
UEFI variable SecureBoot.

Avoid abbreviations in function descriptions.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>