tools: mkeficapsule: allow for specifying GUID explicitly

tools: mkeficapsule: allow for specifying GUID explicitly

The existing options, "--fit" and "--raw," are only used to put a proper
GUID in a capsule header, where GUID identifies a particular FMP (Firmware
Management Protocol) driver which then would handle the firmware binary in
a capsule. In fact, mkeficapsule does the exact same job in creating
a capsule file whatever the firmware binary type is.

To prepare for the future extension, the command syntax will be a bit
modified to allow users to specify arbitrary GUID for their own FMP driver.
OLD:
   [--fit <image> | --raw <image>] <capsule file>
NEW:
   [--fit | --raw | --guid <guid-string>] <image> <capsule file>

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>