xilinx: zynq: Add support for saving env based on bootmode

Enable saving variables to MMC(FAT), NAND, SPI based on primary
bootmode. If bootmode is JTAG, dont save env anywhere(NOWHERE).

Since most of the flashes on zynq evaluation boards are 16MB in size,
set default ENV_OFFSET to 15MB(0xE00000).

Signed-off-by: Ashok Reddy Soma <ashok.reddy.soma@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>