Temporarily increase max number of tasks for small boards

Affected boards: NRF51_DK, NRF52_DK, Beetle. This change is temporary and
will be overwritten by RTX update. It's necessary for the boards to pass
updated tests.

Author: bulislaw

targets/TARGET_ARM_SSG/mbed_rtx.h

@@ -23,7 +23,7 @@
 #define INITIAL_SP              (0x20020000UL)
 #endif
 #ifndef OS_TASKCNT
-#define OS_TASKCNT              6
+#define OS_TASKCNT              7
 #endif
 #ifndef OS_MAINSTKSIZE
 #define OS_MAINSTKSIZE          112

targets/TARGET_NORDIC/mbed_rtx.h

@@ -28,7 +28,7 @@
 #endif
 
 #ifndef OS_TASKCNT
-#define OS_TASKCNT              6
+#define OS_TASKCNT              7
 #endif
 #ifndef OS_MAINSTKSIZE
 #define OS_MAINSTKSIZE          512
@@ -46,7 +46,7 @@
 #define INITIAL_SP              (0x20010000UL)
 #endif
 #ifndef OS_TASKCNT
-#define OS_TASKCNT              6
+#define OS_TASKCNT              7
 #endif
 #ifndef OS_MAINSTKSIZE
 #define OS_MAINSTKSIZE          512