change all versions of input_data() and output_data() to global weak aliases

This changes input_data() and friends from static function to global symbols
under weak alias, to enable board specific overrides (and therefore get rid of
board-specific code in cmd_ide.c)
Also declare ide_bus_offset in the header file, so other files can use
ATA_CURR_BASE as well.

Signed-off-by: Pavel Herrmann <morpheus.ibis@gmail.com>