Makefile: Remove -N ldflag to prevent linker RWX warning

-N option coalesce all sections into single LOAD segment which causes
data and other sections to have executable permission causing warning
with new binutils ld 2.39.
New ld emits warning when any segment have all three permissions RWX.

ld.bfd: warning: test.elf has a LOAD segment with RWX permissions
ld.bfd: warning: fw_dynamic.elf has a LOAD segment with RWX permissions
ld.bfd: warning: fw_jump.elf has a LOAD segment with RWX permissions
ld.bfd: warning: fw_payload.elf has a LOAD segment with RWX permissions

This option was added in below commit -
commit: eeab92f2

 ("Makefile: Convert to a more standard format")

Removing -N option allows to have text and rodata into one LOAD
segment and other sections into separate LOAD segment which prevents
RWX permissions on single LOAD segment. Here X == E

Current
 LOAD           0x0000000000000120 0x0000000080000000 0x0000000080000000
                 0x000000000001d4d0 0x0000000000032ed8  RWE    0x10

-N removed
  LOAD           0x0000000000001000 0x0000000080000000 0x0000000080000000
                 0x00000000000198cc 0x00000000000198cc  R E    0x1000
  LOAD           0x000000000001b000 0x000000008001a000 0x000000008001a000
                 0x00000000000034d0 0x0000000000018ed8  RW     0x1000

Signed-off-by: Rahul Pathak <rpathak@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Tested-by: Samuel Holland <samuel@sholland.org>