triggers/zio-trig-hrt.c: fix fire on re-enable

bug: on re-enable it automatically fires even if no timer was programmed
or it was already expired. Here the most simple procedure to
reproduce the bug:

insmod zio-zero.ko
cd /sys/bus/zio/devices/zzero-0000/zero-input-8
echo hrt > current_trigger
echo 0 > trigger/enable
echo 1 > trigger/enable

the trigger fired and you will find a block in all buffers.

Signed-off-by: Federico Vaga <federico.vaga@gmail.com>
Acked-by: Alessandro Rubini <rubini@gnudd.com>
zio-buffer.h: on store fail set block lost alarm

Signed-off-by: Federico Vaga <federico.vaga@gmail.com>
doc/img: fix color of device and buffer

Signed-off-by: Federico Vaga <federico.vaga@gmail.com>
drivers/zio-zero.c: add gain, offset attributes to sequence32 channel

Signed-off-by: Federico Vaga <federico.vaga@gmail.com>
Makefile: replace EXTRA_CFLAGS with ccflags-y

Signed-off-by: Federico Vaga <federico.vaga@gmail.com>
Makefile: add debug option

Signed-off-by: Federico Vaga <federico.vaga@gmail.com>