kernel: Use hardlinks for do_populate_sysroot for speed

kernel: Use hardlinks for do_populate_sysroot for speed

The kernel tree is large and doesn't need to be copied. Override
the default sysroot handling function to use a hardlink copying
function in python.

This commit also drops the copying of the /lib directory which
just contains the kernel modules. We never use those in the sysroot
so there is little point in carrying those around.

For linux-yocto this takes the do_populate_sysroot time 24s -> 14s.

(From OE-Core rev: 13259459e200a237ca486cbe1123a0b0a4d1eebf)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>