common: cmd_ide: use __weak and add prototypes

clang chokes about the concept of having an alias to an
always_inlined function. gcc likely just ignores the always
inlined since binary sizes are equal before and after this
patch. Convert the aliases to weak functions and provide
missing prototypes.

cc: Pavel Herrmann <morpheus.ibis@gmail.com>
Signed-off-by: Jeroen Hofstee <jeroen@myspectrum.nl>