armv7-m: add bootblock entry point

armv7-m: add bootblock entry point

this adds an entry point jumping to main for the bootblock.

BUG=None
BRANCH=ToT
TEST=Built coreboot for cosmos
Signed-off-by: Daisuke Nojiri <dnojiri@chromium.org>
Change-Id: I1c9ea6ba63a1058e09613d969fe00308260037be
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Original-Commit-Id: 662d0083f25008b55b9bc5fbce9e30e6b80c2c65
Original-Change-Id: I74f2f5e3b3961ab54a7913e6b3a3ab0e6fd813a3
Original-Reviewed-on: https://chromium-review.googlesource.com/225205
Original-Commit-Queue: Daisuke Nojiri <dnojiri@chromium.org>
Original-Tested-by: Daisuke Nojiri <dnojiri@chromium.org>
Original-Reviewed-by: Vadim Bendebury <vbendeb@chromium.org>
Reviewed-on: http://review.coreboot.org/9382
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>