externalsrc: stop rebuilds of 2+ externalsrc recipes sharing the same git repo

externalsrc do_configure task watches oe-devtool-tree-sha1 file and its
checksum. That file basically contains the result of `git add -A
${EXTERNALSRC} && git write-tree` which is the hash of temporary
"commit" of the non committed changes. This file is stored in the .git
directory of the git repo of the externalsrc recipe. do_configure then
depends on the checksum of oe-devtool-tree-sha1 file.

If 2+ recipes with different externalsrc paths but same git repo (e.g.
one recipe at /some/path and the other at /some/path/subdir) are parsed,
this oe-devtool-tree-sha1 will be overwritten by those recipes at
parsing time since .git is shared between those recipes.

If there is one non committed git change in /some/path but not in
/some/path/subdir, the oe-devtool-tree-sha1 of both recipes will be
different.

What will happen is that recipe1 will watch over the
oe-devtool-tree-sha1 with a specific checksum, fill in file-checksums
for do_configure correctly, then recipe2 will watch over the identically
named file with different content also fill in the file-checksums
varflag. When do_configure of recipe1 will be evaluated for
re-execution, oe-devtool-tree-sha1 will be of the value of what is
watched over by recipe2, thus triggering a rebuild of recipe1.

This behavior is not always reproducible which I'm guessing is due to a
small window between recipe1 putting info into oe-devtool-tree-sha1 and
calculating the checksum of that file and recipe2 putting its content
into oe-devtool-tree-sha1.

By appending the name of the recipe to oe-devtool-tree-sha1, we make
sure that a recipe won't have its oe-devtool-tree-sha1 overwritten by
another recipe sharing the same externalsrc git repo.

Signed-off-by: Quentin Schulz <quentin.schulz@streamunlimited.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

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