test/x25519: First test speed, then test implementation
test/x25519: First test speed, then test implementation

Makes it easier to tune implementation speed.

BRANCH=none
BUG=b:62813194
TEST=- Increase CONFIG_RO_SIZE to 60kb
     - Increase console stack size to 2048
     - Define CONFIG_CURVE25519_CORTEXM0 (next patch)
     make BOARD=hammer PROJECT=x25519 TEST_BUILD=y
     ./util/flash_ec --board=hammer --image=build/hammer/x25519.bin
     EC console: runtest

Change-Id: I9097e326a3772b2e05f9b90a68ac715ed467c442
Reviewed-on: https://chromium-review.googlesource.com/542636
Commit-Ready: Nicolas Boichat <f3daba1cdc403799935674d3cd4b0e5bd1804232@chromium.org>
Tested-by: Nicolas Boichat <f3daba1cdc403799935674d3cd4b0e5bd1804232@chromium.org>
Reviewed-by: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>

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