Implement stack overflow checking for the BSP

Previous patches implemented stack overflow checking for the APs.
This patch builds on the BSP stack poisoning patch to implement
stack overflow checking for the BSP, and also prints out maximum
stack usage. It reveals that our 32K stack is ridiculously oversized,
especially now that the lzma decoder doesn't use a giant 16K on-stack
array.

Break the stack checking out into a separate function, which
we will later use for the APs.

CPU0: stack from 00180000 to 00188000:Lowest stack address 00187ad8

To test failure, change the DEADBEEF stack poison value in c_start.S
to something else. Then we should get an error like this:
Stack overrun on BSP.Increase stack from current 32768 bytes
CPU0: stack from 00180000 to 00188000:Lowest stack address 00180000

Separate the act of loading from the act of starting the payload. This
allows us better error management and reporting of stack use. Now we
see:
CPU0: stack from 00180000 to 00188000:Lowest stack address 00187ad8

Tested for both success and failure on Link. At the same time, feel free
to carefully check my manipulation of _estack.

Change-Id: Ibb09738b15ec6a5510ac81e45dd82756bfa5aac2
Signed-off-by: Ronald G. Minnich <rminnich@chromium.org>
Reviewed-on: http://review.coreboot.org/1286
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

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