Tweak/Consolidate followup (#21261)

platformio.ini

 
 # BTT SKR 1.4 needs a UART3 tweak
 [env:LPC1768_btt_skr_v1_4]
-platform    = ${LPC1768.platform}
+platform    = ${env:LPC1768.platform}
 extends     = env:LPC1768
 build_flags = ${env:LPC1768.build_flags} -DLPC_PINCFG_UART3_P4_28