gd32: add new chip series f1x0

GD32F1X0 (X can be 3, 5, 7 and 9) is a series of Cortex-M3 MCUs by
GigaDevice, which features pin-to-pin package compatibility with
STM32F030 MCU line. F150 adds USB support to F130, and F170/F190 adds
CAN support.

Currently the code mainly targets GD32F130 and F150 chips. Some register
are different between F130/150 and F170/190, just like the difference
between STM32F1 Performance line and Connectivity line.

From the perspective of registers and memory map, GD32F1X0 seems like a
mixture between STM32F1 and STM32F0 (because it is designed to be
pin-to-pin compatible with F0, but with Cortex-M3 like F1). A bunch of
code are shared between STM32 and GD32, and these code are specially
processed to include the GD32 headers instead of STM32 headers when meet
GD32F1X0.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>

Reviewed-by: Karl Palsson <karlp@tweak.net.au>
gd32/rcc.[ch] are forks of stm32f1/rcc
gd32/flash.[ch] are forks of stm32f0/flash
No attempts at deduplicating this have been done at this stage.  We can
see where they move in the future.

lib/gd32/f1x0/Makefile

@@ -0,0 +1,47 @@
+##
+## This file is part of the libopencm3 project.
+##
+## Copyright (C) 2019 Icenowy Zheng <icenowy@aosc.io>
+##
+## This library is free software: you can redistribute it and/or modify
+## it under the terms of the GNU Lesser General Public License as published by
+## the Free Software Foundation, either version 3 of the License, or
+## (at your option) any later version.
+##
+## This library 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 Lesser General Public License for more details.
+##
+## You should have received a copy of the GNU Lesser General Public License
+## along with this library.  If not, see <http://www.gnu.org/licenses/>.
+##
+
+LIBNAME		= libopencm3_gd32f1x0
+SRCLIBDIR	?= ../..
+
+PREFIX		?= arm-none-eabi
+
+CC		= $(PREFIX)-gcc
+AR		= $(PREFIX)-ar
+TGT_CFLAGS	= -Os \
+		  -Wall -Wextra -Wimplicit-function-declaration \
+		  -Wredundant-decls -Wmissing-prototypes -Wstrict-prototypes \
+		  -Wundef -Wshadow \
+		  -I../../../include -fno-common \
+		  -mcpu=cortex-m3 $(FP_FLAGS) -mthumb -Wstrict-prototypes \
+		  -ffunction-sections -fdata-sections -MD -DGD32F1X0
+TGT_CFLAGS      += $(DEBUG_FLAGS)
+# ARFLAGS	= rcsv
+ARFLAGS		= rcs
+
+OBJS		= rcc.o flash.o
+
+OBJS            += \
+                   gpio_common_all.o gpio_common_f0234.o \
+		   rcc_common_all.o \
+		   flash_common_all.o flash_common_f.o flash_common_f01.o
+
+VPATH += ../:../../cm3:../common:../../stm32/common
+
+include ../../Makefile.include