feat: add flags to build kernel headers package

Related to #21

README.md

@@ -142,6 +142,7 @@ I would advice to read official documentation, however it's short version:
   * Debian bullseye with edge kernel:    `./compile.sh  BOARD=mkspi BRANCH=edge    RELEASE=bullseye BSPFREEZE=yes BUILD_DESKTOP=no BUILD_MINIMAL=no KERNEL_CONFIGURE=no COMPRESS_OUTPUTIMAGE=sha,gpg,img`
   * Debian bookworm with current kernel: `./compile.sh  BOARD=mkspi BRANCH=current RELEASE=bookworm BSPFREEZE=yes BUILD_DESKTOP=no BUILD_MINIMAL=no KERNEL_CONFIGURE=no COMPRESS_OUTPUTIMAGE=sha,gpg,img`
   * Debian bookworm with edge kernel:    `./compile.sh  BOARD=mkspi BRANCH=edge    RELEASE=bookworm BSPFREEZE=yes BUILD_DESKTOP=no BUILD_MINIMAL=no KERNEL_CONFIGURE=no COMPRESS_OUTPUTIMAGE=sha,gpg,img`
+  * append `INSTALL_HEADERS=yes BUILD_KSRC=yes INSTALL_KSRC=yes` and `BSFREEZE=yes` flags if you will need kernel headers, e.g. to compile custom WiFi drivers
   * if you need to modify the kernel sources: `./compile.sh  BOARD=mkspi BRANCH=current RELEASE=jammy BSPFREEZE=yes BUILD_DESKTOP=no BUILD_MINIMAL=no KERNEL_CONFIGURE=no COMPRESS_OUTPUTIMAGE=sha,gpg,img kernel-patch`
 5. Wait a 20-180 minutes (depends of your hardware, mostly disk system) and check `output\images\` directory