target: add Espressif ESP32 basic support

ESP32 is a dual core Xtensa SoC
Not full featured yet. Some of the missing functionality:
-Semihosting
-Flash breakpoints
-Flash loader
-Apptrace
-FreeRTOS

Signed-off-by: Erhan Kurubas <erhan.kurubas@espressif.com>
Change-Id: I76fb184aa38ab9f4e30290c038b5ff8850060750
Reviewed-on: https://review.openocd.org/c/openocd/+/6989
Tested-by: jenkins
Reviewed-by: Ian Thompson <ianst@cadence.com>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>

src/target/espressif/Makefile.am

@@ -2,5 +2,9 @@ noinst_LTLIBRARIES += %D%/libespressif.la
 %C%_libespressif_la_SOURCES = \
        %D%/esp_xtensa.c \
        %D%/esp_xtensa.h \
+       %D%/esp_xtensa_smp.c \
+       %D%/esp_xtensa_smp.h \
        %D%/esp32s2.c \
-       %D%/esp32s2.h
+       %D%/esp32s2.h \
+       %D%/esp32.c \
+       %D%/esp32.h