toolchain: allow side by side sysroot directories

Currently our toolchain infrastructure assumes that every toolchain has
nested sysroot directories. However that's not true for all of them. The
Codescape toolchains from Imagination Technologies use a side by side
sysroot structure, for instance.

This patch allows our toolchain infrastructure to detect what kind of
sysroot structure we have (nested or side by side) and performs the
appropriate actions.

[Thomas: update the comment above the function, to explain what's
going on with nested sysroots and side-by-side sysroots.]

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>