powerpc/corenet_ds: Update README.srio-boot-corenet

Update some descriptions due to the implementation changes:

For master:
	Get rid of the SRIOBOOT_MASTER build target, and to support
	for serving as a SRIO boot master via environment variable.
For slave:
	1. When compile the slave image for boot from SRIO, no longer
	   need to specify which SRIO port it will boot from.
	2. All slave's cores should be in hold off.

Signed-off-by: Liu Gang <Gang.Liu@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>