at91, omap2plus: configs: migrate CONFIG_ENV_ to defconfigs

Enable the extended ENV options for AT91 and OMAP2PLUS in order to be
able to use CONFIG_ENV_UBI_* on these architectures.

As this change also makes the configs ENV_SIZE, ENV_SECT_SIZE,
ENV_OFFSET visible to AT91 and OMAP2PLUS, migrate users of these to
KConfig.

This migration was run using an extended moveconfig.py which evaluates
expressions such as "(512 << 10)". See patch ("moveconfig: expand
simple expressions").

All modified boards were built with SOURCE_DATE_EPOCH=0 before and
after the change and successfully confirmed that the identical binary
is generated (the only exception was igep00x0, which does not define
CONFIG_ENV_IS_IN_UBI in the original board header. Once that is
defined, the test passes too).

hs: rebased patch to:
  68b90e57: "configs: tinker-rk3288 disable CONFIG_SPL_I2C_SUPPORT"

Signed-off-by: Markus Klotzbuecher <markus.klotzbuecher@kistler.com>
Cc: Heiko Schocher <hs@denx.de>
Cc: Eugen Hristev <eugen.hristev@microchip.com>
Cc: Tom Rini <trini@konsulko.com>