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Fix ARM uclibc build with !BR2_UCLIBC_ARM_BX.

When BR2_UCLIBC_ARM_BX is not set, UCLIBC_OPT_UNSET gets called with an extra
argument.  Remove it.

Signed-off-by: Justin Maggard <jmaggard10@gmail.com>
Acked-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>