linker-defs: add size values for kernel/app memory

These can be computed from start/end values, but such
arithmetic can't be done when populating at build time
struct member values.

Some documentation has been added to explain exactly
what these symbols mean. It is intended for application
RAM to come first, then followed by kernel RAM and then
all unclaimed memory (also considered kernel RAM).

Obsolete _image_ram_all[] removed.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>