tinycbor: exclude Blackfin ADI toolchain

tinycbor: exclude Blackfin ADI toolchain

As tinycbor uses fopencookie() which is not available with the Blackfin
ADI external toolchain (UCLIBC_HAS_GLIBC_CUSTOM_STREAMS is not enabled),
add a dependency on !BR2_TOOLCHAIN_EXTERNAL_BLACKFIN_UCLINUX.

This avoids the following build failure:

src/open_memstream.c: In function ‘open_memstream’:
src/open_memstream.c:105: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘vtable’
src/open_memstream.c:105: error: ‘vtable’ undeclared (first use in this function)
src/open_memstream.c:105: error: (Each undeclared identifier is reported only once
src/open_memstream.c:105: error: for each function it appears in.)
src/open_memstream.c:105: error: expected expression before ‘{’ token
src/open_memstream.c:111: warning: implicit declaration of function ‘fopencookie’
src/open_memstream.c:111: warning: return makes pointer from integer without a cast

Signed-off-by: Fabrice Fontaine <fabrice.fontaine@orange.com>
[Thomas: improve commit message.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>