unix: Make manifest selection match other ports.

Changes are:
- The default manifest.py is moved to the variants directory (it's in
  "boards" in other ports).
- The coverage variant now uses a custom manifest in its variant directory
  to add frzmpy/frzstr.
- The frzmpy/frzstr tests are moved to variants/coverage/.

ports/unix/Makefile

@@ -14,7 +14,7 @@ include ../../py/mkenv.mk
 include $(VARIANT_DIR)/mpconfigvariant.mk
 
 # use FROZEN_MANIFEST for new projects, others are legacy
-FROZEN_MANIFEST ?= manifest.py
+FROZEN_MANIFEST ?= variants/manifest.py
 FROZEN_DIR =
 FROZEN_MPY_DIR =
 

ports/unix/manifest_coverage.py

@@ -1,2 +0,0 @@
-freeze_as_str('coverage-frzstr')
-freeze_as_mpy('coverage-frzmpy')

ports/unix/variants/coverage/manifest.py

@@ -0,0 +1,2 @@
+freeze_as_str("frzstr")
+freeze_as_mpy("frzmpy")

ports/unix/variants/coverage/mpconfigvariant.mk

@@ -11,7 +11,7 @@ CFLAGS += \
 
 LDFLAGS += -fprofile-arcs -ftest-coverage
 
-FROZEN_MANIFEST = manifest_coverage.py
+FROZEN_MANIFEST ?= $(VARIANT_DIR)/manifest.py
 
 MICROPY_ROM_TEXT_COMPRESSION = 1
 MICROPY_VFS_FAT = 1