dualboot: update-firmware: update the check of nand or emmc system

Check the second ('/') and third ('ubifs') field of 'rootfs' entry in
'/proc/mounts' as the first one ('rootfs_a' or 'rootfs_b') may be changed by
custormers:

    root@ccmp15-dvk:~# cat /proc/mounts
    ubi0:rootfs_b / ubifs rw,relatime,assert=read-only,ubi=0,vol=5 0 0
    [...]

https://onedigi.atlassian.net/browse/DEL-8399

Signed-off-by: Tatiana Leon <Tatiana.Leon@digi.com>

meta-digi-dey/recipes-digi/dualboot/dualboot/update-firmware

@@ -31,7 +31,7 @@ ALT_BOOT=""
 ALT_ROOTFS=""
 
 # Check if the rootfs is ubifs to determine if it is a nand or emmc device
-NANDROOTFS="$(sed -ne "s/\(rootfs\).*\<ubifs\>.*/\1/g;T;p" /proc/mounts 2>/dev/null)"
+NANDROOTFS="$(grep -qs '[[:blank:]]\+/[[:blank:]]\+ubifs.*' /proc/mounts 2>/dev/null && echo 1)"
 
 ## Local functions
 usage() {