Remove power button work-around for ARM

On x86 U-Boot cannot see the power button, which means that the EC must
deal with it, and may power off the unit at any time. To get around this,
we write the vbcontext every time we change it.

Since this isn't a problem on ARM, and we want to avoid spurious writes
(due to delay and disk wear), make this code execute only on x86 machines.

BUG=chrome-os-partner:13717
BUG=chrome-os-partner:7689
BRANCH=snow,link
TEST=manual
On snow, see that the EC no longer gets MKBP messages to write the nv
context.
On link, manually add a print to U-Boot's nvstorage_write_disk() function
and see that changing language in recovery still causes a write.

Change-Id: I62508739c9fc3aca46fba58b196a8af45269af2a
Reviewed-on: https://gerrit.chromium.org/gerrit/32464
Commit-Ready: Tom Wai-Hong Tam <waihong@chromium.org>
Reviewed-by: Tom Wai-Hong Tam <waihong@chromium.org>
Tested-by: Tom Wai-Hong Tam <waihong@chromium.org>
2 files changed