dtc: Add Kconfig option to pad device tree blob

This will allow arch(s) that use device tree blobs to pad the end of the
device tree so they can be modified by board files at run time. This will
help prevent errors such as FDT_ERR_NOSPACE from occurring.

Signed-off-by: Eric Schikschneit <eric.schikschneit@novatechautomation.com>
[trini: Change default order so that X86 && EFI_APP works correctly]

doc/develop/makefiles.rst

@@ -1430,10 +1430,13 @@ When kbuild executes, the following steps are followed (roughly):
 	A central rule exists to create `$(obj)/%.dtb` from `$(src)/%.dts`;
 	architecture Makefiles do no need to explicitly write out that rule.
 
+        The device tree can now be padded by the specified number of bytes
+        by setting CONFIG_SYS_DTC_PAD_BYTES instead of explicitly setting
+        DTC_FLAGS with the -p option.
+
 	Example::
 
 		targets += $(dtb-y)
-		DTC_FLAGS ?= -p 1024
 
 7.9 Preprocessing linker scripts
 --------------------------------