tools/mkeficapsule: improve online help

Show short arguments along with long arguments in online help:

    $ tools/mkeficapsule -h
    Usage: mkeficapsule [options] <output file>
    Options:
        -f, --fit <fit image>       new FIT image file
        -r, --raw <raw image>       new raw image file
        -i, --index <index>         update image index
        -I, --instance <instance>   update hardware instance
        -K, --public-key <key file> public key esl file
        -D, --dtb <dtb file>        dtb file
        -O, --overlay               the dtb file is an overlay
        -h, --help                  print a help message

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>