infra: remove usage of 4th parameter to inner-generic-package

In preparation of the removal of the 4th parameter to inner-generic-package
and the pkgparentdir helper function, this patch removes the direct usage of
this 4th parameter. The remaining usage
ifeq ($(4),boot/)
can become
$(filter boot/%,$(pkgdir))
instead (and similar for toolchain).

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>