Add new flags CFLAGS_FOR_BUILD and LDFLAGS_FOR_BUILD

In canadian cross build CFLAGS/LDFLAGS may contain flags good
for "host" but not valid for "build".  Define new flags.  eg.

  # flags common to both "host" and "build"
  CFLAGS_FOR_BUILD="-O2 -s -Wno-error"
  LDFLAGS_FOR_BUILD=

  # additional flags for "host"
  CFLAGS="$CFLAGS_FOR_BUILD $HOST_CFLAGS"
  LDFLAGS="$LDFLAGS_FOR_BUILD $HOST_LDFLAGS"

Change-Id: I723d4fa2b62a3945922a027555f30997a66cce48