ARM: OMAP5+: configs: Fix default boot command

The default boot command searches for dofastboot varaiable
and does a fastboot if it is set to 1.
But the condition "if test ${dofastboot} -eq 1" always
returns true if dofastboot is not defined and breaking mmc boot.
So make dofastboot as 0 by default and let the runtime
environment set it if fastboot is required.

Reported-by: Yan Liu <yan-liu@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>