net: consolidate PXE processor architecture type Kconfig

net: consolidate PXE processor architecture type Kconfig

DHCP and DHCPv6 use the same value defined in
https://www.iana.org/assignments/dhcpv6-parameters#processor-architecture
to encode the processor architecture type. We should only use a single
Kconfig symbol for both protocols.

Furthermore we should make the value customizable. This allows for instance
to choose between "x86 BIOS" or "x64 UEFI".

As "x86 BIOS" is encoded as 0, we should not use this value to switch
off transmission of the DHCP option. Use 0xFF instead.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Tom Rini <trini@konsulko.com>