[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.

src/include/compiler.h

 extern void dbg_hex_dump_da ( unsigned long dispaddr,
 			      const void *data, unsigned long len );
 
-/* Compatibility with existing Makefile */
 #if DEBUG_SYMBOL
-#define DBGLVL DEBUG_SYMBOL
+#define DBGLVL_MAX DEBUG_SYMBOL
+#else
+#define DBGLVL_MAX 0
+#endif
+
+/* Allow for selective disabling of enabled debug levels */
+#if DBGLVL_MAX
+int __debug_disable;
+#define DBGLVL ( DBGLVL_MAX & ~__debug_disable )
+#define DBG_DISABLE( level ) do {				\
+	__debug_disable |= ( (level) & DBGLVL_MAX );		\
+	} while ( 0 )
+#define DBG_ENABLE( level ) do {				\
+	__debug_disable &= ~( (level) & DBGLVL_MAX );		\
+	} while ( 0 )
 #else
 #define DBGLVL 0
+#define DBG_DISABLE( level ) do { } while ( 0 )
+#define DBG_ENABLE( level ) do { } while ( 0 )
 #endif
 
 #define DBGLVL_LOG	1