generate_ptable.sh: fix system partition type
generate_ptable.sh: fix system partition type

the partition type for "system" partition is Linux filesystem (8300)

Signed-off-by: Fathi Boudra <7cd6e55cc07a66bc71d7901a4cddfe6c22aab659@linaro.org>

96boards-hikey/l-loader,96boards-hikey/l-loader