gdb: fix regression in gdb_port command

The gdb_port command can be invoked during normal execution
to report the port used for gdb, whereas it was listed as
CONFIG stage only, which caused an error when excuting
it to return the reported error.

Also in line with the grander goal of making more commands
available during all "modes" (perhaps retiring config mode),
there is no particular reason to limit gdb_port to the
config stage.

Regression was introduced in:

b3bf1d12b2fdfba1c1cbee3e1afbfbb27cbd1a26 aka
v0.4.0-rc1-32-gb3bf1d1

Signed-off-by: Øyvind Harboe <oyvind.harboe@zylin.com>