x86: rename KERNEL_TEXT_SIZE => KERNEL_IMAGE_SIZE

The KERNEL_TEXT_SIZE constant was mis-named, as we not only map the kernel
text but data, bss and init sections as well.

That name led me on the wrong path with the KERNEL_TEXT_SIZE regression,
because i knew how big of _text_ my images have and i knew about the 40 MB
"text" limit so i wrongly thought to be on the safe side of the 40 MB limit
with my 29 MB of text, while the total image size was slightly above 40 MB.

Signed-off-by: Ingo Molnar <mingo@elte.hu>