Handle most LDSCRIPT setting centrally

Currently, some linker scripts are found by common code in config.mk.
Some are found using CONFIG_SYS_LDSCRIPT, but the code for that is
sometimes in arch config.mk and sometimes in board config.mk.  Some
are found using an arch-specific rule for looking in CPUDIR, etc.

Further, the powerpc config.mk rule relied on CONFIG_NAND_SPL
when it really wanted CONFIG_NAND_U_BOOT -- which covered up the fact
that not all NAND_U_BOOT builds actually wanted CPUDIR/u-boot-nand.lds.

Replace all of this -- except for a handful of boards that are actually
selecting a linker script in a unique way -- with centralized ldscript
finding.

If board code specifies LDSCRIPT, that will be used.
Otherwise, if CONFIG_SYS_LDSCRIPT is specified, that will be used.

If neither of these are specified, then the central config.mk will
check for the existence of the following, in order:

$(TOPDIR)/board/$(BOARDDIR)/u-boot-nand.lds (only if CONFIG_NAND_U_BOOT)
$(TOPDIR)/$(CPUDIR)/u-boot-nand.lds (only if CONFIG_NAND_U_BOOT)
$(TOPDIR)/board/$(BOARDDIR)/u-boot.lds
$(TOPDIR)/$(CPUDIR)/u-boot.lds

Some boards (sc3, cm5200, munices) provided their own u-boot.lds that
were dead code, because they were overridden by a CPUDIR u-boot.lds under
the old powerpc rules.  These boards' own u-boot.lds have bitrotted and
no longer work -- these lds files have been removed.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Tested-by: Graeme Russ <graeme.russ@gmail.com>