recipes: Turn ALLOW_EMPTY to be package specific

Fixes QA warnings during parse

ERROR: QA Issue:
/b/kraj/angstrom/sources/meta-opie/recipes-opie/packagegroups/nativesdk-packagegroup-opie-toolchain-host.bb:
Variable ALLOW_EMPTY is set as not being package specific, please fix
this.
ERROR: QA Issue:
/b/kraj/angstrom/sources/meta-opie/recipes-opie/packagegroups/packagegroup-opie-toolchain-target.bb:
Variable ALLOW_EMPTY is set as not being package specific, please fix
this.
ERROR: QA Issue:
/b/kraj/angstrom/sources/meta-opie/recipes-opie/opie-mediaplayer1/opie-mediaplayer1_git.bb:
Variable ALLOW_EMPTY is set as not being package specific, please fix
this.
ERROR: QA Issue:
/b/kraj/angstrom/sources/meta-opie/recipes-opie/opie-mediaplayer1/opie-mediaplayer1_1.2.5.bb:
Variable ALLOW_EMPTY is set as not being package specific, please fix
this.

Signed-off-by: Khem Raj <raj.khem@gmail.com>