efi_loader: add the number of image entries in efi_capsule_update_info

The number of image array entries global variable is required
to support EFI capsule update. This information is exposed as a
num_image_type_guids variable, but this information
should be included in the efi_capsule_update_info structure.

This commit adds the num_images member in the
efi_capsule_update_info structure. All board files supporting
EFI capsule update are updated.

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>