toolchain-external.mk: fix ARCH_SUBDIR calculation

toolchain-external.mk: fix ARCH_SUBDIR calculation

ARCH_SUBDIR is computed based on the value of ARCH_SYSROOT_DIR and
SYSROOT_DIR. For nested toolchains ARCH_SYSROOT_DIR is a subdir of
SYSROOT_DIR, so a sed command like this one...

  sed -r -e "s:^${SYSROOT_DIR}(.*)/$:\1:"

...basically removes the leading SYSROOT_DIR part from ARCH_SYSROOT_DIR.

But, for side-by-side sysroot toolchains ARCH_SYSROOT_DIR and
SYSROOT_DIR are at the same level, so the above sed command doesn't
make any effect.

This patch therefore improves the calculation of ARCH_SUBDIR to
clearly handle the three possible cases:

 - There is a single sysroot, or the selected architecture sysroot is
   the main one (i.e SYSROOT_DIR == ARCH_SYSROOT_DIR). In this case,
   ARCH_SUBDIR is empty.

 - There are side-by-side sysroots, such as
   SYSROOT_DIR=.../sysroot/mips-r2-hard/ and
   ARCH_SYSROOT_DIR=.../sysroot/mipsel-r2-hard/.

 - The arch-sysroot is nested, such as SYSROOT_DIR=.../sysroot and
   ARCH_SYSROOT_DIR=.../sysroot/armv4t/

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
[Thomas: improve the logic to handle the SYSROOT_DIR==ARCH_SYSROOT_DIR
case.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>