boot: android: rework bootargs concatenation

Rework the bootargs concatenation allocating more accurately
the length that is needed.
Do not forget an extra byte for the null termination byte as,
in some cases, the allocation was 1 byte short.

Fixes: 86f4695b ("image: Fix Android boot image support")
Signed-off-by: Nicolas Belin <nbelin@baylibre.com>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Link: https://lore.kernel.org/r/20241217-fix-bootargs-concatenation-v2-3-b2fd7cf4e130@baylibre.com


Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>