siteinfo: Add mechanism to extend siteinfo information from BSP layer

siteinfo: Add mechanism to extend siteinfo information from BSP layer

In order to add a new architecture or sub-architecture to OE, you currently
need to tweak the table in siteinfo.bbclass. This adds a mechanism so this
can be done from a BSP layer. It needs a function definition which needs
a class file but can then be done with something like:

def rp_testfunc2(archinfo, osinfo, targetinfo, d):
    archinfo['testarch'] = "little-endian bit-32"
    osinfo['testos'] = "common-linux"
    targetinfo['mymach-linux'] = "mymach-linux-common"

    return archinfo, osinfo, targetinfo

SITEINFO_EXTRA_DATAFUNCS = "rp_testfunc2"

[YOCTO #8554]

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>