def do_compile(d):
    with open(d.expand('${B}/os-release'), 'w') as f:
        for field in d.getVar('OS_RELEASE_FIELDS').split():
            unquotedFields = d.getVar('OS_RELEASE_UNQUOTED_FIELDS').split()
            value = d.getVar(field)
            if value:
                if field in unquotedFields:
                    value = sanitise_value(value)
                    f.write('{0}={1}\n'.format(field, value))
                else:
                    f.write('{0}="{1}"\n'.format(field, value))

do_compile(d)

def sanitise_value(ver):
    # unquoted fields like VERSION_ID should be (from os-release(5)):
    #    lower-case string (mostly numeric, no spaces or other characters
    #    outside of 0-9, a-z, ".", "_" and "-")
    ret = ver.replace('+', '-').replace(' ','_')
    return ret.lower()