add: private include header
add: macro encapsulation for assertion or error check
add: ESP_FAIL return code documentation in public headers
change: replaced all assertions by NVS_ASSERT_OR_RETURN macro
change: few internal function return values from void to esp_err_t
change: ESP_ERR_NVS_VALUE_TOO_LONG macro comment
New makefile component_wrapper.mk allows some variables to be set
before component.mk is evaluated. This properly fixes problems with
sdkconfig being hard to access in all phases of the build.
Including component_common.mk is no longer necessary and will print a
deprecation warning for components which use it.
Fixes problems with Eclipse trying to build in directories it shouldn't.
This is a breaking change for existing repositories, they need to rename
any component Makefiles to component.mk and rename their references to
$(IDF_PATH)/make/component.mk to $(IDF_PATH)/make/component_common.mk
