make: don’t override TEST_COMPONENTS
When idf_monitor is launched from make, it gets modified `TEST_COMPONENTS`
variable which contains full paths to the test components instead of the
names. This causes `TEST_COMPONENT_PATHS` to be empty and the unit test
app gets built without any test components.
This change introduces an internal `TEST_COMPONENTS_LIST` variable which
gets set either from `COMPONENTS` if `TESTS_ALL` is 1, or from
`TEST_COMOPONENTS` otherwise.
Bootloader makefile is also fixed to avoid propagating `TESTS_ALL` to the
bootloader build step.
See merge request !569
a9ed6d5a24