avr32: fix linking of atstk100x and favr32 boards

When building some avr32 boards out of tree (e.g. O=..) the linker script could
not be found. This patch references the linker script in source tree.

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

board/atmel/atstk1000/config.mk

 PLATFORM_RELFLAGS	+= -ffunction-sections -fdata-sections
 PLATFORM_LDFLAGS	+= --gc-sections
 TEXT_BASE		= 0x00000000
-LDSCRIPT		= $(obj)board/atmel/atstk1000/u-boot.lds
+LDSCRIPT		= $(src)board/atmel/atstk1000/u-boot.lds

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

 PLATFORM_RELFLAGS	+= -ffunction-sections -fdata-sections
 PLATFORM_LDFLAGS	+= --gc-sections
 TEXT_BASE		= 0x00000000
-LDSCRIPT		= $(obj)board/earthlcd/favr-32-ezkit/u-boot.lds
+LDSCRIPT		= $(src)board/earthlcd/favr-32-ezkit/u-boot.lds