spl: unbreak CONFIG_SPL_MULTI_DTB_FIT after fixing CONFIG_OF_EMBED

spl: unbreak CONFIG_SPL_MULTI_DTB_FIT after fixing CONFIG_OF_EMBED

With commit 9bd76b807636 ("spl: make CONFIG_OF_EMBED pass dts through
fdtgrep"), CONFIG_SPL_MULTI_DTB_FIT has been broken because
cmd_fdtgrep was now unknown in scripts/Makefile.spl after moving
it to dts/Makefile. This bug has been introduced with v2018.01.

This patch moves cmd_fdtgrep from dts/Makefile to scripts/Makefile.lib
and includes scripts/Makefile.lib in scripts/Makefile.spl.

Fixes: 9bd76b807636 ("spl: make CONFIG_OF_EMBED pass dts through fdtgrep")
Reported-by: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Simon Goldschmidt <sgoldschmidt@de.pepperl-fuchs.com>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>