Update 'board_f_cpu' to 'board_build.f_cpu'

platformio.ini

@@ -55,7 +55,7 @@ platform      = atmelavr
 framework         = arduino
 board             = megaatmega2560
 build_flags       = ${common.build_flags}
-board_f_cpu   = 16000000L
+board_build.f_cpu = 16000000L
 lib_deps          = ${common.lib_deps}
 src_filter        = ${common.default_src_filter}
 monitor_speed     = 250000
@@ -68,7 +68,7 @@ platform      = atmelavr
 framework         = arduino
 board             = megaatmega1280
 build_flags       = ${common.build_flags}
-board_f_cpu   = 16000000L
+board_build.f_cpu = 16000000L
 lib_deps          = ${common.lib_deps}
 src_filter        = ${common.default_src_filter}
 monitor_speed     = 250000
@@ -149,7 +149,7 @@ monitor_speed = 250000
 [env:LPC1768]
 platform          = nxplpc
 board             = lpc1768
-board_f_cpu     = 100000000L
+board_build.f_cpu = 100000000L
 build_flags       = !python Marlin/src/HAL/HAL_LPC1768/lpc1768_flag_script.py
   ${common.build_flags}
   -DU8G_HAL_LINKS
@@ -172,7 +172,7 @@ monitor_speed   = 250000
 platform          = nxplpc
 #framework        = mbed
 board             = lpc1768
-board_f_cpu    = 100000000L
+board_build.f_cpu = 100000000L
 build_flags       = !python Marlin/src/HAL/HAL_LPC1768/lpc1768_flag_script.py
   ${common.build_flags}
   -DU8G_HAL_LINKS
@@ -227,7 +227,7 @@ platform      = atmelavr
 framework         = arduino
 board             = reprap_rambo
 build_flags       = ${common.build_flags}
-board_f_cpu   = 16000000L
+board_build.f_cpu = 16000000L
 lib_deps          = ${common.lib_deps}
 src_filter        = ${common.default_src_filter}
 monitor_speed     = 250000
@@ -299,7 +299,7 @@ platform      = teensy
 framework         = arduino
 board             = teensy20pp
 build_flags       = ${common.build_flags}
-#board_f_cpu  = 20000000L     ; Bug in Arduino framework disallows boards running at 20Mhz
+#board_build.f_cpu = 20000000L     ; Bug in Arduino framework disallows boards running at 20Mhz
 lib_deps          = ${common.lib_deps}
 lib_ldf_mode      = deep+
 src_filter        = ${common.default_src_filter}