add example demonstrating advanced PlatformIO environment inheritance

platformio_override.sample.ini

@@ -536,3 +536,49 @@ extends = env:esp32dev
 build_flags = ${env:esp32dev.build_flags} -D USERMOD_RF433
 lib_deps = ${env:esp32dev.lib_deps}
   sui77/rc-switch @ 2.6.4
+
+# ------------------------------------------------------------------------------
+# Advanced example: Inheriting all settings from an upstream WLED "release" config,
+# and adding/overriding specific build flags, which is useful if you want to base
+# your environment on a stable WLED release env but easily override some of its
+# defaults, such as the output pin numbers, the default ethernet board ID, etc.
+#
+# WARNING: The WLED project does NOT guarantee that your chosen "upstream" config
+# will exist or be maintained in the future. Environments can sometimes change
+# radically. The compiler will warn you if the upstream configuration doesn't
+# exist anymore, but it won't warn you if it's no longer maintained or has added
+# new flags that may impact your use-case. It is YOUR responsibility to periodically
+# check the status and any new defaults for your chosen upstream configuration.
+# However, it's *generally* safe to pick a well-known, popular release configuration
+# and tweak a few commonly used defaults such as its pin numbers and modes!
+# ------------------------------------------------------------------------------
+
+[env:esp32_eth_custom]
+;; Inherit all settings from upstream config. We can then override specific settings.
+;; WARNING: The upstream environment's name, such as "esp32_eth" in this example,
+;; must also be specified in the other inherited settings (see example below).
+extends = env:esp32_eth
+
+;; Remove conflicting upstream flags to fix compiler "variable redefined" warnings.
+;; WARNING: Must specify the EXACT upstream "build_flags" flag VALUES to unset.
+;; If no value is specified, then ALL values for that flag will be removed, which
+;; also deletes any new values you're trying to set. Also be aware that the
+;; compiler deletes ALL instances of the EXACT VALUES you specify below.
+;; NOTE: If the upstream values are ever changed, the compiler will encounter
+;; duplicate values and complain about "variable redefined" again, meaning that
+;; these values will simply need to be revised. So maintenance is safe and easy.
+build_unflags = ${env:esp32_eth.build_unflags}
+  ;; Unset the old, conflicting flag values:
+  -D WLED_RELEASE_NAME=\"ESP32_Ethernet\"
+  -D RLYPIN=-1
+
+;; Inherit all upstream flags and then overwrite/append with some custom values.
+;; NOTE: You can add/modify ANY of the WLED build-time variables here (it doesn't
+;; have to be values that were in the original upstream config).
+build_flags = ${env:esp32_eth.build_flags}
+  ;; Set/add your new flag values:
+  -D WLED_RELEASE_NAME=\"ESP32_Ethernet_Custom\"
+  -D RLYPIN=15           ;; Relay pin.
+  -D WLED_ETH_DEFAULT=4  ;; "QuinLed-ESP32-Ethernet"
+  -D DATA_PINS=16,3,1,4  ;; LED output channels.
+