Makefile: use long options names for the 'conf' utility

The short option names have been removed as they were starting to
cause too much confusion. See
http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=4062f1a4c030157216dc8932e27131975cf7253c


for details.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>