[linux] Avoid starting currticks() from zero every time

[linux] Avoid starting currticks() from zero every time

iPXE uses currticks() (along with the MAC address(es) of any network
devices) to seed the (non-cryptographic) random number generator.  The
current implementation of linux_currticks() ensures that the first
call to currticks() will always return zero; this results in identical
random number sequences on each run of iPXE on a given machine.  This
can cause odd-looking behaviour due to e.g. the reuse of local TCP
port numbers.

Fix by effectively rounding down the start time recorded by
linux_currticks() to the nearest whole second; this makes it unlikely
that consecutive runs of iPXE will use the exact same RNG sequence.

(Note that none of this affects the cryptographic RNG, which uses
/dev/random as a source of entropy.)

Signed-off-by: Michael Brown <mcb30@ipxe.org>