ESP-IDF platform change only: move to release 5.4ish.

ubxlib needs a later version of ESP-IDF in order to support the new ESP-IDF I2C interface.  We thought this would be ESP-IDF v5.3 but it appears that the ESWP-IDF commit that we need for our I2C API to work properly (992d8bc5f20556adb62571d55a1517a9848d4b51, committed on 24th June 2024) was not included in v5.3 and doesn't appear, yet, to be included in v5.4-dev.  Also it turns out we need a fix applied in commit 8aee667873143a4f2c029d21a4bfff358d287db4 to do with clock stretching, hence we have forked their master branch at commit 6e5414b6c4f265a0adfb56a15fbfbe6beb1f8373 (master on 30th August 2024) and will test with that until the commits we need appears in an ESP-IDF release.

port/platform/common/automation/u_packages.yml

@@ -83,17 +83,29 @@ stm32_cmsis_freertos:
     url: https://github.com/STMicroelectronics/x-cube-freertos.git
     version: v1.1.0
 
+# Note: normally the entry below would point at 
+# https://github.com/espressif/esp-idf and one
+# of their tags, but we need a commit (992d8bc committed
+# on 24th June 2024) that Espressif have added for us,
+# to do with I2C, which, for some reason, they have not
+# yet included in a release (not v5.3, released end August 2024,
+# or even v5.4_dev), and it turns out we need a fix applied
+# in commit 8aee667 to do with clock stretching, hence we
+# have forked their master branch at commit 6e5414b (master
+# branch on 30th August 2024) and created a tag there
+# (v_6e5414b) that we can test with until these things are
+# available in an ESP-IDF release
 esp_idf:
     type: esp_idf
     windows:
-        version: v5.2.2
-        url: https://github.com/espressif/esp-idf
+        version: v_6e5414b
+        url: https://github.com/u-blox/esp-idf
     linux:
-        version: v5.2.2
-        url: https://github.com/espressif/esp-idf
+        version: v_6e5414b
+        url: https://github.com/u-blox/esp-idf
     linux_arm:
-        version: v5.2.2
-        url: https://github.com/espressif/esp-idf
+        version: v_6e5414b
+        url: https://github.com/u-blox/esp-idf
 
 make:
     windows:

port/platform/esp-idf/mcu/esp32/README.md

@@ -6,7 +6,7 @@ Follow the instructions to build for the ESP-IDF platform:
 
 https://docs.espressif.com/projects/esp-idf/en/latest/get-started/index.html#get-started-step-by-step
 
-The builds here are tested with the v5.2.2 release of ESP-IDF from [Github](https://github.com/espressif/esp-idf/releases/tag/v5.2.2) though, since `ubxlib` uses very little of ESP-IDF, versions 4.3 and 4.4 likely continue to work.
+The builds here are tested with ESP-IDF at commit [6e5414b6c4f265a0adfb56a15fbfbe6beb1f8373](https://github.com/espressif/esp-idf/commit/6e5414b6c4f265a0adfb56a15fbfbe6beb1f8373) (`master` on 30th August 2024).  This is after ESP-IDF v5.3: the reason for this peculiar arrangement is that, in order to support the new ESP-IDF I2C interface (introduced in ESP-IDF v5.2), we need commit [992d8bc5f20556adb62571d55a1517a9848d4b51](https://github.com/espressif/esp-idf/commit/992d8bc5f20556adb62571d55a1517a9848d4b51) (commited on 24th June 2024) and that commit was not included in v5.3; it will likely form part of v5.4.  Also it turns out we need a fix applied in commit [8aee667873143a4f2c029d21a4bfff358d287db4](https://github.com/espressif/esp-idf/commit/8aee667873143a4f2c029d21a4bfff358d287db4) to do with clock stretching so, until these commits are available in an ESP-IDF release, we need to test with this intermediate commit.  Since `ubxlib` uses very little of ESP-IDF, versions 4.3 and 4.4 likely continue to work.
 
 # SDK Usage
 You may override or provide conditional compilation flags to ESP-IDF without modifying the build file.  Do this by setting an environment variable `U_FLAGS`, e.g.: