efi_loader: bootmgr: add load option helper functions

In this patch, helper functions for an load option variable (BootXXXX)
are added:
* efi_deserialize_load_option(): parse a string into load_option data
			(renamed from parse_load_option and exported)
* efi_serialize_load_option(): convert load_option data into a string

Those functions will be used to implement efishell command.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Signed-off-by: Alexander Graf <agraf@suse.de>