Merge branch 'bugfix/make_test_components' into 'master'

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