Fix ld error in elf maketarget

Fix ld error in elf maketarget

The sdram_init ELF fails to link:

powerpc64le-linux-gnu-ld -static -nostdlib -T sdram_init.lds \
    --gc-sections -o sdram_init.elf head.o main.o sdram.o console.o \
    libc.o sdram_init.lds
powerpc64le-linux-gnu-ld: error: linker script file 'sdram_init.lds'
    appears multiple times
make: *** [Makefile:70: sdram_init.elf] Error 1

This is because sdram_init.lds is one of the prerequisites, and thus is
contained in $^.  However, it is also explicitly specified as part of
LDFLAGS, as the argument to -T.

Signed-off-by: Boris Shingarov <shingarov@labware.com>