openssl: Use mkhash for STAMP_CONFIGURED

The current way of creating a STAMP_CONFIGURED filename for OpenSSL can
lead to an extremely long filename that makes touch unable to create it,
and fail the build.

Use mkhash to produce a hash against OPENSSL_OPTIONS which creates a
shortert stamp file,

Fixes #572

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>