lsi-image-sim: Add gdb package to Simulation Image

Signed-off-by: John Jacques <john.jacques@lsi.com>
conf/machine: Rename targets to axxiaarm and axxiapowerpc

The original names "elpaso" and "axm5500" refer to targets, and
are mis-leading. The Linux builds, and file system images, are
for either ARM based targets (now axxiaarm) or PowerPC based
targets (now axxiapowerpc).