NVS flash: host-based unit test of nvs::Page

* General tests like page loading from flash
* Rough test of fixed-size data types
* Rough test of blob read
* Added coverage target in cmake, also accessible
  via `idf.py coverage`
* Fixed unsigned comparison in comp. enum table
* introducing temporary LINUX_TARGET define

components/nvs_flash/src/compressed_enum_table.hpp

@@ -35,7 +35,7 @@ public:
 
     Tenum get(size_t index) const
     {
-        assert(index >= 0 && index < Nitems);
+        assert(index < Nitems);
         size_t wordIndex = index / ITEMS_PER_WORD;
         size_t offset = (index % ITEMS_PER_WORD) * Nbits;
 
@@ -44,7 +44,7 @@ public:
 
     void set(size_t index, Tenum val)
     {
-        assert(index >= 0 && index < Nitems);
+        assert(index < Nitems);
         size_t wordIndex = index / ITEMS_PER_WORD;
         size_t offset = (index % ITEMS_PER_WORD) * Nbits;