kbuild: Fix handling of backslashes in *.cmd files

Commit c353acba ("kbuild: make: fix if_changed when command contains
backslashes") attempted to handle backslashes in *.cmd files, but it
only handled double backslashes for some reason. Changing make-cmd to also
handle single backslashes fixes rebuilds with dash, but it breaks bash
again. The reason is that the two shells disagree about the
interpretation of backslash sequences in the echo builtin. The way out
of this is to print the command with printf '%s\n'. While at it,
document what the individual parts of make-cmd do and why.

Reported-and-tested-by: Konstantin Khlebnikov <koct9i@gmail.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>