Document the rationale for the armel build

Call the component xxx_arm5vte, because that's what it does. Explain
"armel", and more generally why this component exists, in a comment.
diff --git a/tests/scripts/all.sh b/tests/scripts/all.sh
index 739f061..e49277a 100755
--- a/tests/scripts/all.sh
+++ b/tests/scripts/all.sh
@@ -1088,9 +1088,14 @@
     make CC=arm-none-eabi-gcc AR=arm-none-eabi-ar LD=arm-none-eabi-ld CFLAGS='-Werror -Wall -Wextra' lib
 }
 
-component_build_arm_none_eabi_gcc_armel () {
+component_build_arm_none_eabi_gcc_arm5vte () {
     msg "build: arm-none-eabi-gcc -march=arm5vte, make" # ~ 10s
     scripts/config.pl baremetal
+    # Build for a target platform that's close to what Debian uses
+    # for its "armel" distribution (https://wiki.debian.org/ArmEabiPort).
+    # See https://github.com/ARMmbed/mbedtls/pull/2169 and comments.
+    # It would be better to build with arm-linux-gnueabi-gcc but
+    # we don't have that on our CI at this time.
     make CC=arm-none-eabi-gcc AR=arm-none-eabi-ar CFLAGS='-Werror -Wall -Wextra -march=armv5te -O1' LDFLAGS='-march=armv5te' SHELL='sh -x' lib
 }