Allow generic_features to handle subdirectories

[zvecr]

Description

Will allow sections like the following to instead use generic:

ifeq ($(strip $(SEND_STRING_ENABLE)), yes)
    OPT_DEFS += -DSEND_STRING_ENABLE
    COMMON_VPATH += $(QUANTUM_DIR)/send_string
    SRC += $(QUANTUM_DIR)/send_string/send_string.c
endif

Which refactors to the single line:

diff --git a/builddefs/generic_features.mk b/builddefs/generic_features.mk
index 261da564b2..1caf447923 100644
--- a/builddefs/generic_features.mk
+++ b/builddefs/generic_features.mk
@@ -34,6 +34,7 @@ GENERIC_FEATURES = \
     PROGRAMMABLE_BUTTON \
     REPEAT_KEY \
     SECURE \
+    SEND_STRING \
     SPACE_CADET \
     SWAP_HANDS \
     TAP_DANCE \

Or,

ifeq ($(strip $(SEQUENCER_ENABLE)), yes)
    OPT_DEFS += -DSEQUENCER_ENABLE
    MUSIC_ENABLE = yes
    SRC += $(QUANTUM_DIR)/sequencer/sequencer.c
    SRC += $(QUANTUM_DIR)/process_keycode/process_sequencer.c
endif

MUSIC_ENABLE ?= no
ifeq ($(MUSIC_ENABLE), yes)
    SRC += $(QUANTUM_DIR)/process_keycode/process_music.c
endif

To just the following dependency logic (plus MUSIC and SEQUENCER lines added to generic_features.mk ):

ifeq ($(strip $(SEQUENCER_ENABLE)), yes)
    MUSIC_ENABLE = yes
endif

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Issues Fixed or Closed by This PR

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).

[zvecr]

I'm thinking e2ce62824bbf5081e6723b11a9a295290f82e9b5 will help with the painter/xap style features? Maybe it will make too much fragmentation? Be it with an more well-defined pattern. But if that's too much for this point i can pull it out and PR later?

[tzarc]

Nah, that works for me.

[zvecr]

Actually scratch that, I want to put some additional ifs round the source file inclusion, and those are being a pain.