Add a fallback case to get_source_date_epoch.sh which reports the modification
time of the script itself in case there is no SCM information available, e.g.
when downloading .tar.gz or .zip tarballs produced by Github.

Also fix the mercurial case while we're at it.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

Signed-off-by: John Crispin <blogic@openwrt.org>

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

SVN-Revision: 48593

SOURCE_DATE_EPOCH is the date of the last modified file using git/svn
as date source.
See https://reproducible-builds.org/specs/source-date-epoch/



Signed-off-by: Alexander Couzens <lynxis@fe80.eu>
Signed-off-by: Felix Fietkau <nbd@openwrt.org>

SVN-Revision: 48584