keystone2: config: restructure handling of default env settings

Currently to customize env for various ks2 boards, individual
variables such as NAME_FS etc are defined and included in the
common config.h to define CONFIG_EXTRA_ENV_SETTINGS. This
doesn't scale well if a variable is not applicable on a
specific board. Using this scheme, we have to define variables
with empty value and it's ugly. Instead, to allow board specific
customization of default env variable, define a common
CONFIG_EXTRA_ENV_KS2_SETTINGS for all common variables and define
board specific variables in individual board specific config.h
using CONFIG_EXTRA_ENV_KS2_BOARD_SETTINGS. Use the common and board
specific variables to define CONFIG_EXTRA_ENV_SETTINGS. This way
more variables can be added in future for individual boards
without affecting the other config.h files.

Signed-off-by: Murali Karicheri <m-karicheri2@ti.com>
Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@ti.com>
Reviewed-by: Tom Rini <trini@ti.com>