distro: deby: Define DEBIAN_SECURITY_UPDATE_MIRROR
distro: deby: Define DEBIAN_SECURITY_UPDATE_MIRROR

Because Debian buster is now in LTS phase, all updates by LTS are
provided through security update repository.
Some of recipes-debian/sources/*.inc now depend on
DEBIAN_SECURITY_UPDATE_MIRROR.

Signed-off-by: Kazuhiro Hayashi <5579fccaa800baf92910671700646e6b27853b34@gmail.com>

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