boards/arm/qemu: enable kernel build for armv7a

[fxysunshine]

Summary

enable qemu-armv7a:knsh to test kernel mode.
See Documentation/platforms/arm/qemu/boards/qemu-armv7a/README.txt for details

Impact

little

Testing

ostest success build with BUILD_KERNEL

   $ ./tools/configure.sh qemu-armv7a:knsh
   $ make V=1 -j7
   $ make export V=1
   $ cd ../apps
   $ ./tools/mkimport.sh -z -x ../nuttx/nuttx-export-*.tar.gz
   $ make import V=1
   $ cd ../nuttx
   $ qemu-system-arm -semihosting -M virt -m 1024 -nographic -kernel ./nuttx

   NuttShell (NSH) NuttX-12.3.0-RC0
   nsh> ps
     PID GROUP PRI POLICY   TYPE    NPX STATE    EVENT     SIGMASK           STACK   USED  FILLED COMMAND
       0     0   0 FIFO     Kthread N-- Ready              0000000000000000 004088 000896  21.9%  Idle_Task
       1     1 100 RR       Kthread --- Waiting  Semaphore 0000000000000000 004040 000304   7.5%  lpwork 0x40119398 0x401193ac
       2     2 100 RR       Task    --- Running            0000000000000000 003032 001032  34.0%  /system/bin/init
   nsh> free
                      total       used       free    largest  nused  nfree
           Kmem:  133058556      16644  133041912  133041152     41      3
           Page:  134217728    1105920  133111808  133111808
   nsh> hello
   Hello, World!!
   nsh> ostest

[xiaoxiang781216]

@fxysunshine please fix the compiler error.