efi_loader: Use ESRT_FW_TYPE_SYSTEMFIRMWARE instead of ESRT_FW_TYPE_UNKNOWN

efi_loader: Use ESRT_FW_TYPE_SYSTEMFIRMWARE instead of ESRT_FW_TYPE_UNKNOWN

We currently set the firmware image type to ESRT_FW_TYPE_UNKNOWN.

The spec defines the following:
ESRT_FW_TYPE_UNKNOWN 0x00000000
ESRT_FW_TYPE_SYSTEMFIRMWARE 0x00000001
ESRT_FW_TYPE_DEVICEFIRMWARE 0x00000002
ESRT_FW_TYPE_UEFIDRIVER 0x00000003

Since we don't support updating DEVICEFIRMWARE or UEFIDRIVER types,
let's switch over to SYSTEMFIRMWARE which seems more appropriate.

Suggested-by: Michal Simek <michal.simek@amd.com>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>