gen_test_keys.sh: Fixup key_index when keys already exist
gen_test_keys.sh: Fixup key_index when keys already exist

BRANCH=none
BUG=chromium:684354
TEST=rm tests/testkeys/key_rsa2048*; make genkeys -j8

Change-Id: I6c75d2d54faf7a02c8fc4ef1ccc8647809c1aae8
Reviewed-on: https://chromium-review.googlesource.com/438949
Commit-Ready: Nicolas Boichat <f3daba1cdc403799935674d3cd4b0e5bd1804232@chromium.org>
Tested-by: Nicolas Boichat <f3daba1cdc403799935674d3cd4b0e5bd1804232@chromium.org>
Reviewed-by: Randall Spangler <62698fdbb84d1779579ee80c3f39fac22017e5bc@chromium.org>
Reviewed-by: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>

coreboot/vboot,coreboot/vboot,coreboot/vboot,coreboot/vboot,coreboot/vboot