Remove DEPENDS argument from add_custom_command(TARGET functions

This raises a warning with recent CMake versions, and isn't required due to the `pico_add_link_depend` already added for these files

Fixes raspberrypi/picotool#235

Author: will-v-pi

tools/CMakeLists.txt

@@ -579,7 +579,6 @@ function(picotool_postprocess_binary TARGET)
         # Embed PT
         if (picotool_embed_pt)
             add_custom_command(TARGET ${TARGET} POST_BUILD
-                DEPENDS ${picotool_embed_pt}
                 COMMAND picotool partition create --quiet ${picotool_embed_pt} $<TARGET_FILE:${TARGET}> $<TARGET_FILE:${TARGET}>
                 VERBATIM)
         endif()
@@ -591,7 +590,6 @@ function(picotool_postprocess_binary TARGET)
             extra_process_args
         )
         add_custom_command(TARGET ${TARGET} POST_BUILD
-            DEPENDS ${picotool_sigfile}
             COMMAND picotool
             ARGS seal
                 --quiet
@@ -604,7 +602,6 @@ function(picotool_postprocess_binary TARGET)
         # Encryption
         if (picotool_aesfile)
             add_custom_command(TARGET ${TARGET} POST_BUILD
-                DEPENDS ${picotool_enc_sigfile} ${picotool_aesfile}
                 COMMAND picotool encrypt --quiet --hash --sign $<TARGET_FILE:${TARGET}> $<TARGET_FILE:${TARGET}> ${picotool_aesfile} ${picotool_enc_sigfile}
                 VERBATIM)
             if (ARGC EQUAL 2)