20210824
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File 'DISTRO_SPECS' has this:

#alternative repo binaries used to build this distro...
ALT1_DISTRO_BINARY_COMPAT='oe'
ALT1_DISTRO_COMPAT_VERSION='dunfell'

This specifies a fallback "compatible distro", which is a fallback if
the package does not exist in the primary compatible distro.
For example, this line in DISTRO_PKGS_SPECS-slackware-15.0:

yes|a52dec|a52dec|exe,dev,doc,nls|compat:

...if a52dec does not exist in the primary compat-distro, then will look
in file 'Packages-${ALT1_DISTRO_BINARY_COMPAT}-${ALT1_DISTRO_COMPAT_VERSION}-official'.

'merge2out' will copy the above file to the build location, via "amd64" "slackware"
"15.0", and will also create a symlink 'packages-${ALT1_DISTRO_BINARY_COMPAT}-${ALT1_DISTRO_COMPAT_VERSION}'
to where the binary packages of the backup distro are located.

There is a catch to this though, the backup binary packages must already be
downloaded. This is achieved by running 'merge2out' and choosing target of
"amd64" "oe" "dunfell", then in the target build folder, run '0setup'
followed by '1download' -- then you will have the binary packages.

