global_data: Ensure we have <config.h> when symbols are not in Kconfig yet

All symbols that are defined in Kconfig will always be defined (or not)
prior to preprocessing due to the -include directive while building.
However, symbols which are not yet migrated will only be defined (or
not) once the board config.h is included, via <config.h>.  While the end
goal must be to migrate all symbols, today we have cases where the size
of gd will get mismatched within the build, based on include order.
Mitigate this by making sure that any <asm/global_data.h> that uses
symbols not in Kconfig does start with <config.h>.  Remove this when not
needed.

Cc: Alexey Brodkin <alexey.brodkin@synopsys.com>
Cc: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Cc: Huan Wang <alison.wang@nxp.com>
Cc: Angelo Dureghello <angelo@sysam.it>
Cc: Rick Chen <rick@andestech.com>
Signed-off-by: Tom Rini <trini@konsulko.com>