SPEAr: Change the default environment variables

SPEAr: Change the default environment variables

This patch modifies the default environment variables as:
1. Default bootargs:
 - console=ttyAMA0,115200
 - For environment present in NOR flash
     root=/dev/mtdblock3
 - For environment present in NAND flash
     root=/dev/mtdblock7
 - Removes "mem=" option
2. Introduces CONFIG_EXTRA_ENV_USBTTY as default usbtty env var even when usbtty
is not selected
3. Add default definitions for nfsboot and ramboot
4. Add a new default environment variable(CONFIG_EXTRA_ENV_UNLOCK) for SPEAr310
and SPEAr320

Signifacance of CONFIG_EXTRA_ENV_USBTTY:
This environment variable is important for flashing utility to work. So if
somebody accidently erases the env sector then also this variable must be
preserved so that flashing utility functions properly.

Signifacance of CONFIG_EXTRA_ENV_UNLOCK:
This env variable is read by the cfi driver to unlock all flash sectors.  This
is necessary because the Parallel NOR flash connected on the spear310 and
spear320 boards, M28W64, has all its sectors in locked state at reset and these
have to be unlocked explicitly before being erased or written.

Signed-off-by: Vipin Kumar <vipin.kumar@st.com>
Signed-off-by: Shiraz Hashim <shiraz.hashim@st.com>
Signed-off-by: Amit Virdi <amit.virdi@st.com>
Acked-by: Stefan Roese <sr@denx.de>
Signed-off-by: Stefan Roese <sr@denx.de>