MIPS: Use the entry point from the ELF file header

In order to fetch the correct entry point with the ISA bit included, for
use by non-ELF boot loaders, parse the output of `objdump -f' for the
start address recorded in the kernel executable itself, rather than
using `nm' to get the value of the `kernel_entry' symbol.

Sign-extend the address retrieved if 32-bit, so that execution is
correctly started on 64-bit processors as well.  The tool always prints
the entry point using either 8 or 16 hexadecimal digits, matching the
address width (aka class) of the ELF file, even in the presence of
leading zeros.

Signed-off-by: Maciej W. Rozycki <macro@mips.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Paul Burton <paul.burton@mips.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/18912/
Signed-off-by: James Hogan <jhogan@kernel.org>

arch/mips/Makefile

@@ -251,20 +251,12 @@ ifdef CONFIG_PHYSICAL_START
 load-y					= $(CONFIG_PHYSICAL_START)
 endif
 
-entry-noisa-y				= 0x$(shell $(NM) vmlinux 2>/dev/null \
-					| grep "\bkernel_entry\b" | cut -f1 -d \ )
-ifdef CONFIG_CPU_MICROMIPS
-  #
-  # Set the ISA bit, since the kernel_entry symbol in the ELF will have it
-  # clear which would lead to images containing addresses which bootloaders may
-  # jump to as MIPS32 code.
-  #
-  entry-y = $(patsubst %0,%1,$(patsubst %2,%3,$(patsubst %4,%5, \
-              $(patsubst %6,%7,$(patsubst %8,%9,$(patsubst %a,%b, \
-              $(patsubst %c,%d,$(patsubst %e,%f,$(entry-noisa-y)))))))))
-else
-  entry-y = $(entry-noisa-y)
-endif
+# Sign-extend the entry point to 64 bits if retrieved as a 32-bit number.
+entry-y		= $(shell $(OBJDUMP) -f vmlinux 2>/dev/null \
+			| sed -n '/^start address / { \
+				s/^.* //; \
+				s/0x\([0-7].......\)$$/0x00000000\1/; \
+				s/0x\(........\)$$/0xffffffff\1/; p }')
 
 cflags-y			+= -I$(srctree)/arch/mips/include/asm/mach-generic
 drivers-$(CONFIG_PCI)		+= arch/mips/pci/