Same as the previous commit.
Move sanity check to prepare1 target to avoid nasty troubles.

Before this commit, LDSCRIPT existence was not checked
when it was specified by CONFIG_SYS_LDSCRIPT.
Now LDSCRIPT existence is checked for all boards.

$(wildcard $(LDSCRIPT)) must point to the linker scripts
with absolute path.
Otherwise, make will terminate with a false error
on out-of-tree build.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>