tests/crypto/test_ecc_dsa intermittently fails on riscv32

[zephyrbot]

Reported by Andrew Boie:

qemu_riscv32              tests/crypto/test_ecc_dsa/test                     FAILED: failed
------------------sanity-out/qemu_riscv32/tests/crypto/test_ecc_dsa/test/qemu.log-------------------
***** BOOTING ZEPHYR OS v1.7.99 - BUILD: Apr 11 2017 19:45:23 *****
tc_start() - TinyCrypt ECC DSA test

  Vector <span>#</span>00 check sig.r - FAILURE
[FAIL] Test <span>#</span>1: ECDSAsign - NIST-p256, SHA2-256
===================================================================
FAIL - main.
===================================================================
PROJECT EXECUTION FAILED
------------------sanity-out/qemu_riscv32/tests/crypto/test_ecc_dsa/test/qemu.log-------------------

This issue was undetected for a while as the test case was only enabled on x86/ARM.
This is an intermittent failure, not 100% reproducible. It may be timing-related.

(Imported from Jira ZEP-2020)

[zephyrbot]

by Jean-Paul Etienne:

Merged in zephyr master
commit b31b416
Issue fixed by commit "riscv32: fixed context restore upon exiting ISR"

Jira: GH-3461

Signed-off-by: Jean-Paul Etienne fractalclone@gmail.com

commit 3486265
riscv32: fixed context restore upon exiting ISR

By now, t0 register restored value is overwritten
by mepc and mstatus values prior to returning from ISR.

Fixed by restoring mstatus and mepc registers before
restoring the caller-saved registers.

As t0 is a temporary register within the riscv ABI,
this issue was unnoticed for most applications, except
for computation intensive apps, like crypto tests.

Signed-off-by: Jean-Paul Etienne fractalclone@gmail.com