Keep custom user flags passed via build_flags, such as a custom LD …

…script (esp8266#4680)

tools/platformio-build.py

@@ -84,24 +84,24 @@ def scons_patched_match_splitext(path, suffixes=None):
 
     LIBSOURCE_DIRS=[
         join(FRAMEWORK_DIR, "libraries")
-    ]
-)
+    ],
 
-env.Replace(
     LINKFLAGS=[
-        "-Os",
-        "-nostdlib",
-        "-Wl,--no-check-sections",
-        "-Wl,-static",
-        "-Wl,--gc-sections",
         "-Wl,-wrap,system_restart_local",
         "-Wl,-wrap,spi_flash_read",
-        "-u", "app_entry",
-        "-u", "_printf_float",
-        "-u", "_scanf_float"
+        "-u", "app_entry"
     ]
 )
 
+# remove LINKFLAGS defined in main.py and keep user custom flags
+try:
+    index = env['LINKFLAGS'].index("call_user_start")
+    if index > 0 and env['LINKFLAGS'][index - 1] == "-u":
+        del env['LINKFLAGS'][index - 1]
+        env['LINKFLAGS'].remove("call_user_start")
+except IndexError:
+    pass
+
 flatten_cppdefines = env.Flatten(env['CPPDEFINES'])
 
 #