autotools: fix autoreconf check

The autoreconf check was incorrect, missing a $ sign to properly
reference a package-specific variable. There was no visible effect
until now since :

 * The existing syntax allowed to access the value defined in the
   package specific .mk file, so when AUTORECONF was set to YES by a
   package, it was working.

 * The default value in Makefile.autotools.in was NO. In fact, when a
   package .mkf file wasn't defining the AUTORECONF variable, the
   Makefile.autotools.in test was testing the empty string against
   'YES', which was false, leading to the AUTORECONF not being done,
   which was the desired effect.

However, in a later patch, we intend to change this default value.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>