Implement generalised RSA public exponents for verified boot

Remove the verified boot limitation that only allows a single
RSA public exponent of 65537 (F4).  This change allows use with
existing PKI infrastructure and has been tested with HSM-based
PKI.

Change the configuration OF tree format to store the RSA public
exponent as a 64 bit integer and implement backward compatibility
for verified boot configuration trees without this extra field.

Parameterise vboot_test.sh to test different public exponents.

Mathematics and other hard work by Andrew Bott.

Tested with the following public exponents: 3, 5, 17, 257, 39981,
50457, 65537 and 4294967297.

Signed-off-by: Andrew Bott <Andrew.Bott@ipaccess.com>
Signed-off-by: Andrew Wishart <Andrew.Wishart@ipaccess.com>
Signed-off-by: Neil Piercy <Neil.Piercy@ipaccess.com>
Signed-off-by: Michael van der Westhuizen <michael@smart-africa.com>
Cc: Simon Glass <sjg@chromium.org>