make ide_led() a weak alias

Make ide_led() a weak alias instead of global/local function/empty macro
based on CONFIG_IDE_LED value and/or board-specific CONFIGs, to get rid of
board-specific code in cmd_ide.c
Define dummy values to get rid of compoler errors in case where ide_led()
used to be an empty macro

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