nrf: Use -O1 for debug builds

[arturo182]

While O0 is great for debugging, the produced binary doesn't fit on the
feather52 anymore.

[microbuilder]

Fair enough. I haven't used GDB on nRF52 CP yet, but good catch!

[dhalbert]

We use -Os for both debug and regular builds on the M0/M4. gdb seems fine with this. We also use -flto, which saves a lot of space. And we set -DNDEBUG even on the debug builds. It appears to me to mostly just turn off some assertions.

[arturo182]

gdb is fine with -Os, but some variables might get optimized out, so I'd prefer to use the least compression possible while still being able to build and flash. It's a good balance.