brcm63xx: image: trim revision code used for --rsa-signature

The imagetag RSA signature field may not exceed 20 characters, so trim the
Git hash portion from the REVISION variable in order to ensure that the
resulting string fits within this field.

The current revision values may be longer than 20 chars, especially when
building within a dirty tree. In this case, the revision will look like
"r2435+75-b4aa3c89" which, combined with the "LEDE-" prefix, is 21 chars long.

Fixes the following error spotted by the buildbots:

    Error: RSA Signature (rsa_signature,r) too long.
    make[4]: *** [.../lede-brcm63xx-generic-NEUFBOX6-squashfs-cfe.bin] Error 1

Signed-off-by: Jo-Philipp Wich <jo@mein.io>