package/pkg-generic: postpone evaluation of dependency conditions

package/pkg-generic: postpone evaluation of dependency conditions

In the pkg-inner macros, all variables, but the positional arguments,
must be $$-prefixed, so that they are expanded only when the macro is
evaluated in each package, not when the macro is parsed.

It is to be noted, though, that the current code, even though
incorrect by the above rules, seemed to work. However, the upcoming
addition of download dependencies, mimicking that code, would not work
unless it was $$-prefixed.

So, for consistency sake, and for correctness sake, let's always use
the $$-prefix in the inner macro.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>