[compiler] Allow for selective disabling of debug levels at runtime

[compiler] Allow for selective disabling of debug levels at runtime

The usefulness of DBGLVL_IO is limited by the fact that many cards
require large numbers of uninteresting I/O reads/writes at device
probe time, typically when driving a bit-bashing I2C/SPI bus to read
the MAC address.

This patch adds the DBG_DISABLE() and DBG_ENABLE() macros, which can
be used to temporarily disable and re-enable selected debug levels.
Note that debug levels must still be enabled in the build in order to
function at all: you can't use DBG_ENABLE(DBGLVL_IO) in an object
built with DEBUG=object:1 and expect it to do anything.