efm32: set safe minimum working area

The smallest available RAM size for this family is 2K, set this as the
default. Issue reported by quitte on IRC.

Change-Id: I3318f7f268f7681ffe2cddab61820f4b94c4e5fd
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/1559
Tested-by: jenkins

tcl/target/efm32_stlink.cfg

@@ -9,11 +9,11 @@ if { [info exists CHIPNAME] } {
 }
 
 # Work-area is a space in RAM used for flash programming
-# By default use 16kB
+# By default use 2kB
 if { [info exists WORKAREASIZE] } {
    set _WORKAREASIZE $WORKAREASIZE
 } else {
-   set _WORKAREASIZE 0x4000
+   set _WORKAREASIZE 0x800
 }
 
 if { [info exists CPUTAPID] } {