Global: Allow overriding float-abi flags

We currently default to "-mfloat-abi=hard -mfpu=fpv4-sp-d16" for M4F cores, and
and variations of "-mfloat-abi=soft" for the others. Keep the M4F default, and
move others to no FP flags for consistency, but allow overriding these flags
via the FP_FLAGS environment variable.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>

lib/lm4f/Makefile

@@ -2,6 +2,7 @@
 ## This file is part of the libopencm3 project.
 ##
 ## Copyright (C) 2009 Uwe Hermann <uwe@hermann-uwe.de>
+## Copyright (C) 2013 Alexandru Gagniuc <mr.nuke.me@gmail.com>
 ##
 ## 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
@@ -19,8 +20,9 @@
 
 LIBNAME		= libopencm3_lm4f
 
+FP_FLAGS	?= -mfloat-abi=hard -mfpu=fpv4-sp-d16
 PREFIX		?= arm-none-eabi
-#PREFIX		?= arm-elf
+
 CC		= $(PREFIX)-gcc
 AR		= $(PREFIX)-ar
 CFLAGS		= -Os -g \
@@ -28,7 +30,7 @@ CFLAGS		= -Os -g \
 		  -Wredundant-decls -Wmissing-prototypes -Wstrict-prototypes \
 		  -Wundef -Wshadow \
 		  -I../../include -fno-common \
-		  -mcpu=cortex-m4 -mthumb -mfloat-abi=hard -Wstrict-prototypes \
+		  -mcpu=cortex-m4 -mthumb $(FP_FLAGS) -Wstrict-prototypes \
 		  -ffunction-sections -fdata-sections -MD -DLM4F
 # ARFLAGS	= rcsv
 ARFLAGS		= rcs