ARM: make decompressor debug output user selectable

Make the decompressor debug output user selectable, otherwise merely
enabling DEBUG_LL causes the decompressor to become board specific,
thereby preventing a multi-platform kernel from booting.  Enabling
DEBUG_LL doesn't cause the kernel itself to become platform specific
unless EARLY_PRINTK is enabled, or one of the debugging routines is
added in a path that results in it being called.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>