Fix loading kernel from official vendor firmware

Fix loading kernel from official vendor firmware

As it turned out, some old kernel versions from Atheros LSDK expect
different arguments passed from bootloader: RAM size (in bytes) in 3rd
parameter and FLASH size (in MB) in 4th.

Most of TP-Link firmware also contains kernel which follows this
'broken' approach.

To make fix more universal, load kernel with different parameters and
their values based on 'lsdk_kernel' environment variable. If it's > 0,
RAM and FLASH size will be passed to satisfy LSDK based kernel
requirements.