configs: k2*_evm: let each board decide env location

Not all TI Keystone2 EVMs want environment in NAND flash.
K2G EVM which has an MMC/SD slot, keep environment in a
FAT partition on SD card.

Since ti_armv7_keystone2.h defines environment is in NAND,
boards which do not follow that have to #undef'ine that
configuration. This leads to ugly ordering issues around
where exactly the include of ti_armv7_keystone2.h can come
in within the k2*_evm.h files.

Move environment location to config file of each board.
This should make it easy to change it for any one board
without affecting all other boards.

Signed-off-by: Sekhar Nori <nsekhar@ti.com>