cmake: LD: Specify the entry point in the linker scripts

The entry point can and therefore should be set by linker
scripts. Whenever possible one should express things in the source
language, be it .c or .ld, and not in code generators or in the build
system.

This patch removes the flag -eCONFIG_KERNEL_ENTRY from the linker's
command line and replaces it with the linker script command

ENTRY(CONFIG_KERNEL_ENTRY)

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>