make: fix buildsizes* targets

- also ignore BOARD_INSUFFICIENT_RAM boards
- unify/make work BOARD filter

Not sure why it failed I don't know the `<<<` construct .. I've got a `GNU bash, version 4.3.18(1)-release (x86_64-unknown-linux-gnu)`
The reason for the current form is that I did not get any more readable version to work - if you provide a working patch I'll happily change it.
In particular:
```diff
--- a/Makefile.buildtests
+++ b/Makefile.buildtests
@@ -37,8 +37,9 @@ buildtest:
        \
        for BOARD in $(BOARD_BLACKLIST); do \
                echo "Ignoring $${BOARD} (blacklisted)"; \
-               BOARDS=$$(echo \ $${BOARDS}\  | sed -e 's/ '$${BOARD}' / /'); \
+               BOARDS=$$(echo " $${BOARDS} " | sed -e "s/ $${BOARD} / /"); \
        done; \
+       echo RESULT: \'$${BOARDS}\'; exit 1;\
        \
        rm -rf "$$BINDIRBASE"; \
        for BOARD in $${BOARDS}; do \
```
results in
```
RESULT: ' msb-430h z1 redbee-econotag msba2 qemu-i386 mbed_lpc1768 telosb avsextrem arduino-due msb-430 native wsn430-v1_3b pttu stm32f0discovery wsn430-v1_4 udoo chronos stm32f4discovery '
```
for `examples/rpl_udp`.