powerpc: Set entry point and text address in linker script

Currently we set the kernel entry point and the address of the text
section in the Makefile, using CONFIG_KERNEL_START.

But we've already got <asm/page.h> in the linker script, so we can just
use KERNELBASE directly. That means if we ever change KERNELBASE there's
one less place to change it.

And we can set the entry point with ENTRY().

There are zero differences from "readelf -a vmlinux" with or without this
patch.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>