config/toolchain: hide sysroot name when in backend mode

In backend mode, setting the sysroot name is the
responsibility of the upper-layer build system.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

reporter_name="Yann E. MORIN"

reporter_url="http://ymorin.is-a-geek.org/projects/crosstool"

reporter_comment="Experimental toolchain for ARM cortex-m3 in Thumb-only, noMMU, and newlib."