pkg-generic: fix fallout of <pkg>_STRIP_COMPONENTS introduction

The introduction of <pkg>_STRIP_COMPONENTS broke the build of the
target tar package, because support/dependencies/check-host-tar.mk
defines TAR_STRIP_COMPONENTS to --strip-components. Which leads to
have the package infrastructure do:

 $$(TAR_STRIP_COMPONENTS)=$$($(2)_STRIP_COMPONENTS)

which for the tar package evaluates to:

 $$(TAR_STRIP_COMPONENTS)=$$(TAR_STRIP_COMPONENTS)

which evalutes to:

 --strip-components=--strip-components

Which obviously doesn't work really well. And in fact the
TAR_STRIP_COMPONENTS definition in
support/dependencies/check-host-tar.mk is no longer necessary: it was
needed in the days where we were trying to support old tar versions
that did not support --strip-components. But nowadays, when such an
old tar version is encountered, we build our own host-tar which
supports --strip-components.

Fixes:

  http://autobuild.buildroot.org/results/ae2/ae20df67f99f75b1ba5d5b7316ad265d66f3aa66/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>