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>
diff --git a/.travis.yml b/.travis.yml
index 8fa0b34..39ae19c 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -15,6 +15,7 @@
           - 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:
diff --git a/tests/scripts/all.sh b/tests/scripts/all.sh
index 7570b11..e103c9a 100755
--- a/tests/scripts/all.sh
+++ b/tests/scripts/all.sh
@@ -2396,7 +2396,7 @@
     scripts/config.py baremetal
     # This is an imperfect substitute for
     # component_build_arm_linux_gnueabi_gcc_arm5vte
-    # in case the gcc-arm-linux-gnueabihf toolchain is not available
+    # in case the gcc-arm-linux-gnueabi toolchain is not available
     make CC="${ARM_NONE_EABI_GCC_PREFIX}gcc" AR="${ARM_NONE_EABI_GCC_PREFIX}ar" CFLAGS='-std=c99 -Werror -Wall -Wextra -march=armv5te -O1' LDFLAGS='-march=armv5te' SHELL='sh -x' lib
 
     msg "size: ${ARM_NONE_EABI_GCC_PREFIX}gcc -march=armv5te -O1"