cmd: move CMD_PXE to Kconfig

Currently, CMD_PXE is forcibly enabled in config_distro_defaults.h, so
that general purpose distributions can rely on it being defined. This
header is included, under conditions or not, by various archs or
famillies of archs / SoCs.

However, it is very possible that boards based on those SoCs will not
have a physical ethernet connector at all, even if the have a MAC; for
example, the Nanopi Neo AIR (sunxi H3) does not. It is also possible
that network booting is absolutely not necessary for a device.

However, it is not possible to disable the PXE command, as it is
forcibly enabled and is non-configurable.

But it turns out we already have a config option to build a distro-ready
image, in the name of DISTRO_DEFAULTS.

Move CMD_PXE out of the hard-coded config_distro_defaults.h into a
Kconfig option, that gets selected by DISTRO_DEFAULTS when it is set.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Joe Hershberger <joe.hershberger@ni.com>
[trini: Make it select MENU, run moveconfig.py]
Signed-off-by: Tom Rini <trini@konsulko.com>