Restructure the locking around SPI transfers
Reduce excessive FMU stack usage
Update sensors tests
Merge pull request #1131 from PX4/pr-1131

Fix SPI struct unitialized use warning
A more compatible way of checking for an empty string.
Add quotes around the variable. It's safer.
Merge pull request #1154 from sjwilks/submodule_tool

Make the check for an empty string more compatible with linux distros
Merge pull request #1148 from PX4/fmu_stack

Reduce excessive FMU stack usage