Makefile.am: fix override of target 'check-recursive'

To prevent executing the Jim Tcl tests, the makefile's target
'check-recursive' has been overridden in commit 56d163ce7951
("jimtcl: update to 0.77, the current version, enable only
specific modules").
This causes a runtime warning during build:
	Makefile:6332: warning: overriding recipe for target 'check-recursive'
	Makefile:5098: warning: ignoring old recipe for target 'check-recursive'

Instead of override the makefile's target 'check-recursive',
prevent the recursion by re-assigning as empty the variable
SUBDIRS for this specific target only.

Change-Id: I03d1c467eba42316a59aeed4612d6bdbe6211282
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Fixes: 56d163ce7951 ("jimtcl: update to 0.77, the current version, enable only specific modules")
Reviewed-on: http://openocd.zylin.com/5986
Tested-by: jenkins