boot: android: fix extra command line support

boot: android: fix extra command line support

Check that the value at the address kcmdline_extra is not 0
instead of checking the address value itself keeping it
consistent with what is done for kcmdline.

Fixes: b36b227b ("android: boot: support extra command line")
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Signed-off-by: Nicolas Belin <nbelin@baylibre.com>
Link: https://lore.kernel.org/r/20241217-fix-bootargs-concatenation-v2-1-b2fd7cf4e130@baylibre.com
Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>