common/xyzModem.c: Do not use hard-coded address for debug buffer

Under the plethora of #ifdefs, the xyzModem code hid this pearl:
static char *zm_out = (char *) 0x00380000;
This was only enabled when DEBUG is defined, so it's probably why it
went unnoticed for so long. No idea what platform had memory at that
exact location, but the this approach is extremely hacky.
Use a static buffer instead.

Signed-off-by: Alexandru Gagniuc <alex.g@adaptrum.com>