external-toolchain: adjust check for uClibc static toolchains

The current check for uClibc toolchain was verifying that a
ld-uClibc.so dynamic loader was present. However, with static-only
uClibc toolchains, this does not work. Instead, we check for an
uClibc-specific header file in the sysroot.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>