common/env_embedded.c: rename PPCENV/PPCTEXT macros

The environment has pretty much nothing to do with just "PPC", so
rename the macros to just __UBOOT_ENV_SECTION__ which is more
readable.

In addition, only a single macro is needed: the environment now goes
either to the default section (USE_HOSTCC is defined) or in the .text
section.

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