xuser-account: Drop allarch inherit

* has build time dependency on TUNE_PKGARCH recipes:
  bitbake-diffsigs sstate-diff/1383303628/*/all*/xuser-account/*do_configure*
  Hash for dependent task base-passwd_3.5.26.bb.do_populate_sysroot changed from 09b3168865b6212269d4e5512a963b9e to ffa22dc51e42a164a7392ec719151629
  Hash for dependent task shadow_4.1.4.3.bb.do_populate_sysroot changed from f426f0c93b2d341050b2547804645314 to 89bdbb151f8c200553c9a74e89504457
  Hash for dependent task shadow-sysroot_4.1.4.3.bb.do_populate_sysroot changed from c42103fe54ac5109eab8de4fb0eb9a75 to f83f628e766fdbd49eb2a44cf6134339

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
packagegroup.bbclass: Drop build-time dependencies

* RFC because:
  packagegroup.bbclass inherits allarch and by default sets
  PACKAGE_ARCH = "all", but then it has runtime-dependency on
  TUNE_PKGARCH packages and different signatures, so instead
  of setting INHIBIT_DEFAULT_DEPS in addition to inherit allarch
  I would like to drop default allarch completely.
* drop virtual/kernel dependency from packagegroup-core-boot
  because kernel isn't needed to build packagegroup and building
  it doesn't influence if it's included in image or not.

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>