RFC: handling of machine/distro features stuff

[Marcin Juszkiewicz <openembedded@hrw.one.pl>]

Currently OpenEmbedded use overrides based on DISTRO, MACHINE, TARGET_ARCH 
and few others. I want to suggest start of using also 
{MACHINE,DISTRO}_FEATURES because we have some stuff in metadata which 
make sense only on some targets only - for example madwifi is limited to 
PCI devices only.

Why do this at all? Let me take wpa-supplicant 0.5.x as example.

I'm building own distribution for my PDA devices and want to use latest 
possible software. But wpa-supplicant 0.5.1 depends on madwifi which 
cannot be build because my target device does not have PCI bus support. If 
I remove madwifi stuff from wpa-supplicant recipe then I will get working 
version. But this cannot be pushed back into repo because NSLU2 related 
distros want version with madwifi support.

Solution can be adding checking for pci support in 
{MACHINE,DISTRO}_FEATURES and only then enable madwifi support:

DEPENDS = "gnutls \ 
${@base_contains("DISTRO_FEATURES", "pci", "madwifi", "",d)}"

Same stuff can go into madwifi recipe so it will get skipped if target 
device lack PCI support.


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