package/apitrace: fix microblaze build failure

package/apitrace: fix microblaze build failure

Apitrace is affected by Gcc Bug 68485 that has already been worked
around, but after version bumping, CMakeLists.txt adds -O2 to
CMAKE_C_FLAGS_RELWITHDEBINFO that is in order appended to
CMAKE_C_FLAGS, making -O0 end appending to fail. To prevent this we
need to avoid CMAKE_C_FLAGS_RELWITHDEBINFO to be appended to
CMAKE_C_FLAGS, so let's do the same as flare-engine package that set
CMAKE_BUILD_TYPE to a fake build type called "buildroot", this way -O2
won't be appended after -O0 and work around restart to work as
expected.

Fixes:
http://autobuild.buildroot.net/results/2e3/2e31abd7b115d4c29117ca82007fd4f87f853ff7/

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>