tegra: override compiler flags for low level init code

Override -march setting for tegra to -march=armv4t for files that are
necessary for low level init on tegra.

The recent change to use -march=armv7-a for armv7 caused a regression
on tegra because tegra starts boot on a arm7tdmi processor before
transferring control to the cortex-a9.  While still executing on the
arm7tdmi there are calls to getenv_ulong() and memset() that cause an
illegal instruction exception if compiled for armv7.

Signed-off-by: Allen Martin <amartin@nvidia.com>
Tested-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>