Introduce generic link section.h symbol files

We create a separate header file for link symbols defined by the link
scripts. It is helpful to have these all in one place and try to
make them common across architectures. Since Linux already has a similar
file, we bring this in even though many of the symbols there are not
relevant to us.

Each architecture has its own asm/sections.h where symbols specifc to
that architecture can be added. For now everything except AVR32 just
includes the generic header.

One change is needed in arch/avr32/lib/board.c to make this conversion
work.

Reviewed-by: Tom Rini <trini@ti.com> (version 5)
Signed-off-by: Simon Glass <sjg@chromium.org>