esp32/boards: Reduce flash usage of ESP32-C6 boards.

Reduces ESP32_GENERIC_C6 application flash size from 2024432 to
1813216 (206KB smaller).

Also has benefit of reducing D/IRAM size, increasing free memory at
runtime (167187 to 148584, -18603 bytes).

Most of this savings comes from building with -Os instead of -O2,
but about 10KB comes from using the SPI flash functions from the ROM.

This work was funded through GitHub Sponsors.

Signed-off-by: Angus Gratton <angus@redyak.com.au>

ports/esp32/boards/ESP32_GENERIC_C6/mpconfigboard.cmake

@@ -2,6 +2,7 @@ set(IDF_TARGET esp32c6)
 
 set(SDKCONFIG_DEFAULTS
     boards/sdkconfig.base
+    boards/sdkconfig.riscv
     boards/sdkconfig.c6
     boards/sdkconfig.ble
 )

ports/esp32/boards/M5STACK_NANOC6/mpconfigboard.cmake

@@ -2,6 +2,7 @@ set(IDF_TARGET esp32c6)
 
 set(SDKCONFIG_DEFAULTS
     boards/sdkconfig.base
+    boards/sdkconfig.riscv
     boards/sdkconfig.c6
     boards/sdkconfig.ble
 )

ports/esp32/boards/UM_TINYC6/mpconfigboard.cmake

@@ -2,6 +2,7 @@ set(IDF_TARGET esp32c6)
 
 set(SDKCONFIG_DEFAULTS
     boards/sdkconfig.base
+    boards/sdkconfig.riscv
     boards/sdkconfig.c6
     boards/sdkconfig.ble
     boards/UM_TINYC6/sdkconfig.board

ports/esp32/boards/sdkconfig.c6

@@ -1,2 +1,10 @@
 # Workaround for https://github.com/espressif/esp-idf/issues/14456
 CONFIG_ESP_SYSTEM_HW_STACK_GUARD=n
+
+# 802.15.4 not currently supported in MicroPython, disabling saves
+# a little compile time (no difference in binary)
+CONFIG_IEEE802154_ENABLED=n
+
+# Using the SPI flash implementation in ROM saves about 10KB of binary size
+# (and some static RAM)
+CONFIG_SPI_FLASH_ROM_IMPL=y

ports/esp32/boards/sdkconfig.riscv

@@ -0,0 +1,3 @@
+# ESP RISC-V binary sizes are generally larger than Xtensa ones,
+# so switch to size optimization by default
+CONFIG_COMPILER_OPTIMIZATION_SIZE=y