core: add a variable that points to the package's hash file

core: add a variable that points to the package's hash file

When a package has a version selection (e.g. Qt5), the licensing terms
may be different across versions, but lie in similarly named files (e.g.
'LICENSE').

However, when we check a file, all the hashes for it must match. So, we
can't have the hashes for two different content of the same file. We
overcame that limitation in the legal-license-file macro, which checks
whether a package has a .hash file in a versioned subdir.

For consistency, we would like to also store the source hashes in that
per-version subdir.

Rather than reconstruct the path to the hash file everywhere we need it,
add a variable that points to it.

Existing users will be converted over in followup patches.

Note: the check for a missing hash file is done in the check-hash helper
script, so this variable must always yield a filename, even of a missing
file, thus we do not use $(wildcard...) to resolve the hash file path;
we use $(wildcard...) only to check if the versioned .hash file exists.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Luca Ceresoli <luca@lucaceresoli.net>
Cc: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>