gdb: Conditionally run GDB script logic for dynamically or statically positioned...

gdb: Conditionally run GDB script logic for dynamically or statically positioned GRUB

There are broadly two classes of targets to consider when loading symbols
for GRUB, targets that determine where to load GRUB at runtime
(dynamically positioned) and those that do not (statically positioned).
For statically positioned targets, symbol loading is determined at link
time, so nothing more needs to be known to load the symbols. For
dynamically positioned targets, such as EFI targets, at runtime symbols
should be offset by an amount that depends on where the runtime chose to
load GRUB.

It is important to not load symbols statically for dynamic targets
because then when subsequently loading the symbols correctly one must
take care to remove the existing static symbols, otherwise there will be
two sets of symbols and GDB seems to prefer the ones loaded first (i.e.
the static ones).

Use autoconf variables to generate a gdb_grub for a particular target,
which conditionally run startup code depending on if the target uses
static or dynamic loading.

Signed-off-by: Glenn Washburn <development@efficientek.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>