common/env_embedded.c: drop support for CONFIG_SYS_USE_PPCENV

CONFIG_SYS_USE_PPCENV is no longer used anywhere. It was used to put
the environment in the special .ppcenv section, but the last
architecture using this section (SuperH) has been changed to not use
it.

Therefore, this commit drops support for CONFIG_SYS_USE_PPCENV
entirely. We only handle two cases:

 - We're building the host tool tools/envcrc, in which case the
   environment is place with no special section attribute (so it
   depends up in .data)

 - We're building U-Boot itself, in which case the environnement is
   placed in the .text section.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>