boards/arm/qemu: enable kernel build for armv7a

See Documentation/platforms/arm/qemu/boards/qemu-armv7a/README.txt for details

Signed-off-by: fangxinyong <fangxinyong@xiaomi.com>

Documentation/platforms/arm/qemu/boards/qemu-armv7a/README.txt

@@ -32,6 +32,35 @@ Getting Started
      -net none -chardev stdio,id=con,mux=on -serial chardev:con \
      -mon chardev=con,mode=readline -kernel ./nuttx
 
+2. knsh
+------
+  This is a configuration of testing the BUILD_KERNEL configuration
+  $ cd nuttx
+  $ ./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> uname -a
+  NuttX 12.3.0-RC0 28dee592a3-dirty Oct 12 2023 03:03:07 arm qemu-armv7a
+  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> /system/bin/hello
+  Hello, World!!
+  nsh>
+
 Debugging with QEMU
 ===================