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>
4e19cbc553