[U-Boot] [RFC] ARMv7: Build cache_v7.c with -O1 to avoid gcc6 breakage

[Hans de Goede]

Hi All,

Here is a workaround for a problem I've been debugging today, for
details see: https://bugzilla.redhat.com/show_bug.cgi?id=1318788

We're still investigating what the real problem is in the mean time
I wanted to share this workaround in case others are hitting the same
problem.

Regards,

Hans