hammer: Increase hook stack size
hammer: Increase hook stack size

Adding entropy takes up to 1028 bytes of stack, let's increase
the stack size to 1280 bytes.

BRANCH=none
BUG=b:38487027
TEST=Flash hammer. On host, reboot hammer to RO:
     usb_updater2 -r; sleep 0.5; usb_updater2 -s
     usb_updater2 -e (adds entropy)
     EC console: check that rollbackinfo shows secret is updated

Change-Id: I7e2d506e0fcc3152d27ac1796db95df6b1a931d1
Reviewed-on: https://chromium-review.googlesource.com/513808
Commit-Ready: Nicolas Boichat <f3daba1cdc403799935674d3cd4b0e5bd1804232@chromium.org>
Tested-by: Nicolas Boichat <f3daba1cdc403799935674d3cd4b0e5bd1804232@chromium.org>
Reviewed-by: Randall Spangler <62698fdbb84d1779579ee80c3f39fac22017e5bc@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec