set git tag for the SDK (raspberrypi#1630)

with this change we can use PICO_SDK_FETCH_GIT_TAG to specify a release tag for the Pico SDK to download.

example usage:
```
set(PICO_SDK_FETCH_FROM_GIT true)
set(PICO_SDK_FETCH_FROM_GIT_TAG "1.5.1")
include(pico_sdk_import.cmake)
```

Author: ahmetkizilay

external/pico_sdk_import.cmake

@@ -18,9 +18,20 @@ if (DEFINED ENV{PICO_SDK_FETCH_FROM_GIT_PATH} AND (NOT PICO_SDK_FETCH_FROM_GIT_P
     message("Using PICO_SDK_FETCH_FROM_GIT_PATH from environment ('${PICO_SDK_FETCH_FROM_GIT_PATH}')")
 endif ()
 
+if (DEFINED ENV{PICO_SDK_FETCH_FROM_GIT_TAG} AND (NOT PICO_SDK_FETCH_FROM_GIT_TAG))
+    set(PICO_SDK_FETCH_FROM_GIT_TAG $ENV{PICO_SDK_FETCH_FROM_GIT_TAG})
+    message("Using PICO_SDK_FETCH_FROM_GIT_TAG from environment ('${PICO_SDK_FETCH_FROM_GIT_TAG}')")
+endif ()
+
+if (PICO_SDK_FETCH_FROM_GIT AND NOT PICO_SDK_FETCH_FROM_GIT_TAG)
+  set(PICO_SDK_FETCH_FROM_GIT_TAG "master")
+  message("Using master as default value for PICO_SDK_FETCH_FROM_GIT_TAG")
+endif()
+
 set(PICO_SDK_PATH "${PICO_SDK_PATH}" CACHE PATH "Path to the Raspberry Pi Pico SDK")
 set(PICO_SDK_FETCH_FROM_GIT "${PICO_SDK_FETCH_FROM_GIT}" CACHE BOOL "Set to ON to fetch copy of SDK from git if not otherwise locatable")
 set(PICO_SDK_FETCH_FROM_GIT_PATH "${PICO_SDK_FETCH_FROM_GIT_PATH}" CACHE FILEPATH "location to download SDK")
+set(PICO_SDK_FETCH_FROM_GIT_TAG "${PICO_SDK_FETCH_FROM_GIT_TAG}" CACHE FILEPATH "release tag for SDK")
 
 if (NOT PICO_SDK_PATH)
     if (PICO_SDK_FETCH_FROM_GIT)
@@ -34,14 +45,14 @@ if (NOT PICO_SDK_PATH)
             FetchContent_Declare(
                     pico_sdk
                     GIT_REPOSITORY https://github.com/raspberrypi/pico-sdk
-                    GIT_TAG master
+                    GIT_TAG ${PICO_SDK_FETCH_FROM_GIT_TAG}
                     GIT_SUBMODULES_RECURSE FALSE
             )
         else ()
             FetchContent_Declare(
                     pico_sdk
                     GIT_REPOSITORY https://github.com/raspberrypi/pico-sdk
-                    GIT_TAG master
+                    GIT_TAG ${PICO_SDK_FETCH_FROM_GIT_TAG}
             )
         endif ()