bitbake: parse/ast: add support for 'built-in' fragments

When reviewing proposed fragments to add settings for DISTRO and MACHINE,
RP noted that such fragments only add clutter and overhead, and there's
no need to maintain them as separate files.

Rather when bitbake sees 'fragmentvar/fragmentvalue' it can expand that into
FRAGMENTVAR = "fragmentvalue".

To achieve that, 'addfragments' directive is extended with a parameter
that sets the name of the variable that holds definitions of such
built-in fragments, for example like this:

"machine:MACHINE distro:DISTRO"

Then each enabled fragment name is matched against these definitions and the
respective variable is set, e.g. 'machine/qemuarm' would match
'machine:MACHINE' and result in MACHINE set to 'qemuarm'.

This happens before any fragment files are looked up on disk,
and no such lookup happens if there was a match, which should prevent
possible misuse of the feature. So the builtin fragment definition
is also an allowlist for them.

Please also see the patches for oe-core that show an application of the feature.

(Bitbake rev: 3b9d7bea915dc7f10e845854f1dae325743f9456)

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

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