[Keymap] Plain60 rgb (#5104)

* Fix compile error if not used with VIA

* Add keymap with support for RGB

* Integrate changes from PR code review

keyboards/plain60/keymaps/RGB/rules.mk

@@ -0,0 +1,23 @@
+#Copyright 2019 Sebastian Spindler <sebastian.spindler@gmail.com>
+
+#This program is free software: you can redistribute it and/or modify
+#it under the terms of the GNU General Public License as published by
+#the Free Software Foundation, either version 2 of the License, or
+#(at your option) any later version.
+
+#This program is distributed in the hope that it will be useful,
+#but WITHOUT ANY WARRANTY; without even the implied warranty of
+#MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+#GNU General Public License for more details.
+
+#You should have received a copy of the GNU General Public License
+#along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# Build Options
+#   comment out to disable the options.
+#
+BOOTMAGIC_ENABLE = no	# Virtual DIP switch configuration(+1000)
+NKRO_ENABLE = yes		# USB Nkey Rollover - if this doesn't work, see here: https://github.com/tmk/tmk_keyboard/wiki/FAQ#nkro-doesnt-work
+RGBLIGHT_ENABLE = yes   # Enable keyboard underlight functionality (+4870)
+RAW_ENABLE = no
+DYNAMIC_KEYMAP_ENABLE = no