Fix unwanted build step for nobuild target

(cherry picked from commit 466d83a)

Author: valeros

builder/main.py

@@ -213,7 +213,7 @@ def get_esptoolpy_reset_flags(resetmethod):
 # Target: Build executable and linkable firmware or SPIFFS image
 #
 
-target_elf = env.BuildProgram()
+target_elf = None
 if "nobuild" in COMMAND_LINE_TARGETS:
     target_elf = join("$BUILD_DIR", "${PROGNAME}.elf")
     if set(["uploadfs", "uploadfsota"]) & set(COMMAND_LINE_TARGETS):
@@ -222,6 +222,7 @@ def get_esptoolpy_reset_flags(resetmethod):
     else:
         target_firm = join("$BUILD_DIR", "${PROGNAME}.bin")
 else:
+    target_elf = env.BuildProgram()
     if set(["buildfs", "uploadfs", "uploadfsota"]) & set(COMMAND_LINE_TARGETS):
         target_firm = env.DataToBin(
             join("$BUILD_DIR", env.get("SPIFFSNAME", "spiffs")), "$PROJECTDATA_DIR")