mkeficapsule: Add support for setting OEM flags in capsule header

Add support for setting OEM flags in the capsule header. As per the
UEFI specification, bits 0-15 of the flags member of the capsule
header can be defined per capsule GUID.

The oemflags will be used for the FWU Multi Bank update feature, as
specified by the Dependable Boot specification[1]. Bit
15 of the flags member will be used to determine if the
acceptance/rejection of the updated images is to be done by the
firmware or an external component like the OS.

[1] - https://git.codelinaro.org/linaro/dependable-boot/mbfw/uploads/6f7ddfe3be24e18d4319e108a758d02e/mbfw.pdf



Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Acked-by: Etienne Carriere <etienne.carriere@linaro.org>