makefile: Add READELF command variable

This allows setting READELF=llvm-readelf in order to use the LLVM
version of the readelf utility. It also aligns with the practice of not
using $(CROSS_COMPILE) in any build recipes directly, reducing the
number of places where $(CROSS_COMPILE) is used.

Signed-off-by: Sam Edwards <CFSworks@gmail.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
diff --git a/Makefile b/Makefile
index 05a9381..fb5cd67 100644
--- a/Makefile
+++ b/Makefile
@@ -406,6 +406,7 @@
 STRIP		= $(CROSS_COMPILE)strip
 OBJCOPY		= $(CROSS_COMPILE)objcopy
 OBJDUMP		= $(CROSS_COMPILE)objdump
+READELF		= $(CROSS_COMPILE)readelf
 LEX		= flex
 YACC		= bison
 AWK		= awk
@@ -2177,7 +2178,7 @@
 # ARM relocations should all be R_ARM_RELATIVE (32-bit) or
 # R_AARCH64_RELATIVE (64-bit).
 checkarmreloc: u-boot
-	@RELOC="`$(CROSS_COMPILE)readelf -r -W $< | cut -d ' ' -f 4 | \
+	@RELOC="`$(READELF) -r -W $< | cut -d ' ' -f 4 | \
 		grep R_A | sort -u`"; \
 	if test "$$RELOC" != "R_ARM_RELATIVE" -a \
 		 "$$RELOC" != "R_AARCH64_RELATIVE"; then \