arm:ensure read_tsc_64 always returns a valid value.

Before this change overflowing the free running clock counter
between the time the timer was read and the time the overflow
check was done resulted in read_tsc_64 returning a to high value.

Change-Id: I1022f271213647f720477c4121d45f0c965456c6
arm_sdimage.sh: use native minix partition util

	. make filesystems on primary partitions to make it
	  easier on the minix partition util
	. so sfdisk isn't required as an external tool
	. also make partition and hitherto x86-only utilities
	  arch-independent

Change-Id: Ib4e07e45d038d084dde1858917d3deb7f0616532