ath79: Fix fallback to bootloader cmdline on empty DT bootargs

ath79: Fix fallback to bootloader cmdline on empty DT bootargs

The MIPS code is supposed to fall back to u-boots bootargs whenever the
/chosen/bootargs property is missing. But this feature was accidentally
disabled when the boot_command_line was initialized with an empty space
just to work around problems with early_init_dt_scan_chosen.

But this feature is necessary for some boards which have a dualboot
mechanism and whose u-boot is calculating the correct partition at runtime
without writing this information back to the u-boot-env.

Signed-off-by: Sven Eckelmann <sven@narfation.org>