TI: Add, use a DEFAULT_LINUX_BOOT_ENV environment string

To deal with a reoccurring problem properly we need to specify addresses
for the Linux kernel, Flatted Device Tree and ramdisk that obey the
constraints within the kernel's Documentation/arm/Booting file but also
make sure that we relocate things within a valid address range.

It is possible with these addresses to also set fdt_high and initrd_high
to the value of 0xffffffff.  We don't do this by default to allow for
the most likely success of people using custom addresses however.

Signed-off-by: Tom Rini <trini@ti.com>