nand_spl: read environment early, when booting from NAND using nand_spl

Currently, when booting from NAND using nand_spl, in the beginning the default
environment is used until later in boot process the dynamic environment is read
out. This way environment variables that must be interpreted early, like the
baudrate or "silent", cannot be modified dynamically and remain at their
default values. Fix this problem by reading out main and redundand (if used)
copies of the environment in the nand_spl code.

Signed-off-by: Guennadi Liakhovetski <lg@denx.de>
Signed-off-by: Scott Wood <scottwood@freescale.com>