Why I did it
For trident4/tomahawk4, linux_ngknet.ko and linux_ngknetcb.ko have to be installed. Also, the kernel modules to load on such chips are different from existing ones, so we add an option is_ltsw_chip to determine the kernel modules to load. The option is_ltsw_chip is controlled by adding 'is_ltsw_chip=1' to platform_env.conf or not.
How to verify it
We verified that existing platforms still work after this change; and for platforms with trident4/tomahawk4, we can load the different kernel modules as expected after adding 'is_ltsw_chip=1' to platform_env.conf
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