package/pkg-generic.mk: increase precision of timestamps

package/pkg-generic.mk: increase precision of timestamps

Currently, the timestamps that we keep in build-time.log use a
second-level precision. However, as we are going to introduce a new
type of graph to draw the time line of a build, this precision is
going to be insufficient, as a number of steps are so short that they
are not even one second long, and generally the rounding to the second
gives a not so great looking graph.

Therefore, we add to the timestamps the nanoseconds using the %N date
specifier. A milli-second precision would have been sufficient, but %N
is all what date(1) provides at the sub-second level.

Since this is changing the format of the build-time.log file, this
commit adjusts the support/scripts/graph-build-time script
accordingly, to account for the floating point numbers that we have as
timestamps.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>