download: handle possibly invalid local tarballs

download: handle possibly invalid local tarballs

Currently it's assumed, that already downloaded tarballs are always
fine, so no checksum checking is performed and the tarball is used even
if it might be corrupted.

From now on, we're going to always check the downloaded tarballs before
considering them valid.

Steps to reproduce:

 1. Remove cached tarball

   rm dl/libubox-2020-08-06-9e52171d.tar.xz

 2. Download valid tarball again

   make package/libubox/download

 3. Invalidate the tarball

   sed -i 's/PKG_MIRROR_HASH:=../PKG_MIRROR_HASH:=ff/' package/libs/libubox/Makefile

 4. Now compile with corrupt tarball source

   make package/libubox/{clean,compile}

Signed-off-by: Petr Štetiar <ynezz@true.cz>