elf: Do not process invalid tunable format

Tunable definitions with more than one '=' on are parsed and enabled,
and any subsequent '=' are ignored.  It means that tunables in the form
'tunable=tunable=value' or 'tunable=value=value' are handled as
'tunable=value'.  These inputs are likely user input errors, which
should not be accepted.

Checked on x86_64-linux-gnu.
Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>