package: enable jobserver for recursive make

Add '+' prefix to the $($(PKG)_BUILD_CMDS) and $($(PKG)_INSTALL*_CMDS)
commands to enable jobserver for the sub-make.

Without the '+' prefix GNU make does not detect the sub-make so it
disable the jobserver for the sub-make.

>From GNU make documentation:
Using the MAKE variable has the same effect as using a ‘+’ character
at the beginning of the recipe line.  This special feature is only
enabled if the MAKE variable appears directly in the recipe: it does
not apply if the MAKE variable is referenced through expansion of
another variable. In the latter case you must use the ‘+’ token to get
these special effects.

Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>