unix: Fix behaviour of COPT/NDEBUG for unix variants.

Based on eg 1e6fd9f2b4, it's understood that
the intention for unix builds is that regular builds disable assert, but
the coverage build should set -O0 and enable asserts.

It looks like this didn't work (even before variants were introduced, eg at
v1.11) -- coverage always built with -Os and -DNDEBUG.

This commit makes it possible for variants to have finer-grained control
over COPT flags, and enables assert() and -O0 on coverage builds.

Other variants already match the defaults so they have been updated.

ports/unix/Makefile

@@ -44,10 +44,18 @@ CFLAGS += $(INC) $(CWARN) -std=gnu99 -DUNIX $(CFLAGS_MOD) $(COPT) -I$(VARIANT_DI
 
 # Debugging/Optimization
 ifdef DEBUG
-CFLAGS += -g
-COPT = -O0
+COPT ?= -O0
 else
-COPT = -Os -fdata-sections -ffunction-sections -DNDEBUG
+COPT ?= -Os
+COPT += -fdata-sections -ffunction-sections
+COPT += -DNDEBUG
+endif
+
+# Always enable symbols -- They're occasionally useful, and don't make it into the
+# final .bin/.hex/.dfu so the extra size doesn't matter.
+CFLAGS += -g
+
+ifndef DEBUG
 # _FORTIFY_SOURCE is a feature in gcc/glibc which is intended to provide extra
 # security for detecting buffer overflows. Some distros (Ubuntu at the very least)
 # have it enabled by default.

ports/unix/variants/coverage/mpconfigvariant.mk

@@ -1,6 +1,7 @@
 PROG ?= micropython-coverage
 
-COPT = -O0
+# Disable optimisations and enable assert() on coverage builds.
+DEBUG ?= 1
 
 CFLAGS += \
 	-fprofile-arcs -ftest-coverage \

ports/unix/variants/fast/mpconfigvariant.mk

@@ -1,6 +1,6 @@
 # build synthetically fast interpreter for benchmarking
 
-COPT = "-O2 -DNDEBUG -fno-crossjumping"
+COPT += "-fno-crossjumping -O2"
 
 PROG = micropython-fast
 

ports/unix/variants/minimal/mpconfigvariant.mk

@@ -1,6 +1,4 @@
 # build a minimal interpreter
-COPT = -Os -DNDEBUG
-
 PROG = micropython-minimal
 
 FROZEN_MANIFEST =