Fix missing dependency on Travis

Was getting errors like:

In file included from /usr/include/limits.h:25:0,
                 from /usr/lib/gcc-cross/arm-linux-gnueabi/5/include-fixed/limits.h:168,
                 from /usr/lib/gcc-cross/arm-linux-gnueabi/5/include-fixed/syslimits.h:7,
                 from /usr/lib/gcc-cross/arm-linux-gnueabi/5/include-fixed/limits.h:34,
                 from ../include/mbedtls/check_config.h:30,
                 from ../include/mbedtls/build_info.h:81,
                 from common.h:26,
                 from asn1write.c:20:
/usr/include/features.h:367:25: fatal error: sys/cdefs.h: No such file or directory

There are two packages to choose from: armhf or armel. Since the comment
in all.sh says we're trying to be close to Debian's "armel"
architecture, choose that, and fix a comment that was mentioning
gnueabihf for no apparent reason.

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>

.travis.yml

@@ -15,6 +15,7 @@ jobs:
           - gcc-arm-none-eabi
           - libnewlib-arm-none-eabi
           - gcc-arm-linux-gnueabi
+          - libc6-dev-armel-cross
       language: python # Needed to get pip for Python 3
       python: 3.5 # version from Ubuntu 16.04
       install: