spl: make CONFIG_OF_EMBED pass dts through fdtgrep

Building spl with CONFIG_OF_EMBED enabled results in an error message
on my board: "SPL image too big". This is because the fdtgrep build
step is only executed for CONFIG_OF_SEPARATE.

Fix this by moving the fdtgrep build step ('cmd_fdtgreo') from
scripts/Makefile.spl to dts/Makefile so that the reduced dtb is
available for all kinds of spl builds.

The resulting variable name for the embedded device tree blob changes,
too, which is why common.h and fdtdec.c have tiny changes.

Signed-off-by: Simon Goldschmidt <sgoldschmidt@de.pepperl-fuchs.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/include/common.h b/include/common.h
index e14e1da..6e24545 100644
--- a/include/common.h
+++ b/include/common.h
@@ -201,6 +201,7 @@
 extern ulong monitor_flash_len;
 int mac_read_from_eeprom(void);
 extern u8 __dtb_dt_begin[];	/* embedded device tree blob */
+extern u8 __dtb_dt_spl_begin[];	/* embedded device tree blob for SPL/TPL */
 int set_cpu_clk_info(void);
 int mdm_init(void);
 int print_cpuinfo(void);