kbuild: Fix module signature generation

Rusty had clearly not actually tested his module signing changes that I
(trustingly) applied as commit e2a666d5 ("kbuild: sign the modules
at install time"). That commit had multiple bugs:

 - using "${#VARIABLE}" to get the number of characters in a shell
   variable may look clever, but it's locale-dependent: it returns the
   number of *characters*, not bytes. And we do need bytes.

   So don't use "${#..}" expansion, do the stupid "wc -c" thing instead
   (where "c" stands for "bytes", not "characters", despite the letter.

 - Rusty had confused "siglen" and "signerlen", and his conversion
   didn't set "signerlen" at all, and incorrectly set "siglen" to the
   size of the signer, not the size of the signature.

End result: the modified sign-file script did create something that
superficially *looked* like a signature, but didn't actually work at
all, and would fail the signature check. Oops.

Tssk, tssk, Rusty.

But Rusty was definitely right that this whole thing should be rewritten
in perl by somebody who has the perl-fu to do so.  That is not me,
though - I'm just doing an emergency fix for the shell script.

Cc: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>