avr32: fix linking

This patch fixes following error:

---8<---
avr32-linux-ld: --gc-sections and -r may not be used together
--->8---

Since 8aba9dce all avr32 boards are broken due
to linking error as seen above.

Signed-off-by: Andreas Bießmann <biessmann@corscience.de>

arch/avr32/config.mk

@@ -26,4 +26,4 @@ CROSS_COMPILE ?= avr32-linux-
 CONFIG_STANDALONE_LOAD_ADDR ?= 0x00000000
 
 PLATFORM_RELFLAGS	+= -ffixed-r5 -fPIC -mno-init-got -mrelax
-PLATFORM_LDFLAGS	+= --relax
+LDFLAGS_u-boot		= --gc-sections --relax

board/atmel/atngw100/config.mk

 CONFIG_SYS_TEXT_BASE		= 0x00000000
 PLATFORM_RELFLAGS	+= -ffunction-sections -fdata-sections
-PLATFORM_LDFLAGS	+= --gc-sections

board/atmel/atstk1000/config.mk

 PLATFORM_RELFLAGS	+= -ffunction-sections -fdata-sections
-PLATFORM_LDFLAGS	+= --gc-sections
 CONFIG_SYS_TEXT_BASE		= 0x00000000

board/earthlcd/favr-32-ezkit/config.mk

 PLATFORM_RELFLAGS	+= -ffunction-sections -fdata-sections
-PLATFORM_LDFLAGS	+= --gc-sections
 CONFIG_SYS_TEXT_BASE		= 0x00000000

board/mimc/mimc200/config.mk

 CONFIG_SYS_TEXT_BASE		= 0x00000000
 PLATFORM_RELFLAGS	+= -ffunction-sections -fdata-sections
-PLATFORM_LDFLAGS	+= --gc-sections

board/miromico/hammerhead/config.mk

 CONFIG_SYS_TEXT_BASE		= 0x00000000
 PLATFORM_RELFLAGS	+= -ffunction-sections -fdata-sections
-PLATFORM_LDFLAGS	+= --gc-sections