[OE-core] Consistency and use cases for IMAGE_FSTYPES

[Tom Rini]

Hey all,

Over in meta-ti I kicked off a discussion
(https://lists.yoctoproject.org/pipermail/meta-ti/2012-March/000779.html)
about if we should be using '?=' or '+=' with IMAGE_FSTYPES in the
machine conf files.  This has been discussed a little bit before
(http://thread.gmane.org/gmane.comp.handhelds.openembedded.core/2060/focus=2061).
 The problem is we have the following and I believe ultimately
conflicting use cases:
- The machine needs to say 'I need or support the following formats'
- The distro needs to say 'I always want format X'
- The user needs to say 'I know best, give me only format X'
- The user needs to say 'I know best, give me what you support + X'

We achieve this today, but not very nicely.  For the last one you need
to start playing order of operations games and that's just not nice.
My suggestion is that we need to change what the machine.conf sets to
another variable (SOMETHING_IMAGE_FSTYPES), and a default
IMAGE_FSTYPES becomes ?= ${SOMETHING_IMAGE_FSTYPES}.  Distros, and
users can then work more easily with their use cases.  Comments?

-- 
Tom