esp32: Refactor backtrace and add esp_backtrace_print()

This commit refactors backtracing within the panic handler so that a common
function esp_backtrace_get_next_frame() is used iteratively to traverse a
callstack.

A esp_backtrace_print() function has also be added that allows the printing
of a backtrace at runtime. The esp_backtrace_print() function allows unity to
print the backtrace of failed test cases and jump back to the main test menu
without the need reset the chip. esp_backtrace_print() can also be used as a
debugging function by users.

components/unity/component.mk

@@ -9,6 +9,10 @@ endif
 COMPONENT_ADD_INCLUDEDIRS = include unity/src
 COMPONENT_SRCDIRS = unity/src .
 
+ifdef CONFIG_UNITY_ENABLE_BACKTRACE_ON_FAIL
+COMPONENT_PRIV_INCLUDEDIRS += include/priv
+endif
+
 ifndef CONFIG_UNITY_ENABLE_IDF_TEST_RUNNER
 COMPONENT_OBJEXCLUDE += unity_runner.o
 endif