am335x_evm: Make SPL_OS also check the boot_os variable for falcon mode

We use the same variable as a3m071 in the environment to determine if we
should boot into Linux or U-Boot.  This is useful on boards like
Beaglebone Black or AM335x GP EVM where we have persistent storage for
the environment.

Signed-off-by: Tom Rini <trini@ti.com>