support/download: add possibility to not fail on missing hash

In very constrained cases, it might be needed to not fail if a hash is
missing. This is notably the case for custom external toolchains to be
downloaded, because we do have a .hash file for external toolchains,
but we obviously can not have hashes for all existing custom toolchains
(he, "custom"!).

So, add a way to avoid failing in that case.

>From the Makefile, we export the list of files for which not to check
the hash. Then, from the check-hash script, if no check was done, and
the file we were trying to match in in this exclusion list, we just exit
without error.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Gustavo Zacarias <gustavo@zacarias.com.ar>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Tested-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

changes v6 -> v7:
  - /beautify/ the pattern in the case clause

Changed v5 -> v6:  (Arnout)
  - fix the pattern in the case clause

Changes v4 -> v5:
  - micro-optimisation, use case-esac instead of a for-loop  (Arnout)
  - typoes  (Arnout)

Changes v3 -> v4:
  - drop the magic value, use a list of excluded files  (Arnout)

Changes v1 -> v2:
  - fix typoes in commit log

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Tested-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>