powerpc/corenet_ds: Get rid of the SRIOBOOT_MASTER build target

Get rid of the SRIOBOOT_MASTER build target, and to support for serving as
a SRIO boot master via environment variable. Set the environment variable
"bootmaster" to "SRIO1" or "SRIO2" using the following command:

		setenv bootmaster SRIO1
		saveenv

The "bootmaster" will enable the function of the SRIO boot master, and
this has the following advantages compared with SRIOBOOT_MASTER build
configuration:
	1. Reduce a build configuration item in boards.cfg file.
	   No longer need to build a special image for master, just use a
	   normal target image and set the "bootmaster" variable.
	2. No longer need to rebuild an image when change the SRIO port for
	   boot from SRIO, just set the corresponding value to "bootmaster"
	   based on the using SRIO port.

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