Changed the line 54 and the image is built correctly flashed and working on the device.
below is the log:
./create_signed_image.sh
Generating CWE for rootfs mdm9x28-image-minimal-swi-mdm9x28-wp.ubi
CWE Header Generator 3.00. Copyright (C) 2012 Sierra Wireless, Inc.
Output CWE Header File ./temp.mbn.hdr
Image size 0x1a00000, crc32 0xe3d69c76
53248+1 records in
53248+1 records out
27263376 bytes (27 MB, 26 MiB) copied, 0.0467859 s, 583 MB/s
Generating CWE for bootloader appsboot.mbn
CWE Header Generator 3.00. Copyright (C) 2012 Sierra Wireless, Inc.
Output CWE Header File ./temp.mbn.hdr
Image size 0x8a528, crc32 0xc9861f6
1107+1 records in
1107+1 records out
566968 bytes (567 kB, 554 KiB) copied, 0.000979026 s, 579 MB/s
Generating CWE for kernel boot-yocto-mdm9x28.4k.img
CWE Header Generator 3.00. Copyright (C) 2012 Sierra Wireless, Inc.
Output CWE Header File ./temp.mbn.hdr
Image size 0x952570, crc32 0x46136ada
19091+1 records in
19091+1 records out
9774848 bytes (9.8 MB, 9.3 MiB) copied, 0.0179286 s, 545 MB/s
Creating Top-level CWE header. Type = APPL
CWE Header Generator 3.00. Copyright (C) 2012 Sierra Wireless, Inc.
Output CWE Header File ./temp.mbn.hdr
Image size 0x23dcf48, crc32 0x806dc4f1
Concatenating files to create: demoyocto.cwe
0+1 records in
0+1 records out
400 bytes copied, 2.1219e-05 s, 18.9 MB/s
73447+1 records in
73447+1 records out
37605192 bytes (38 MB, 36 MiB) copied, 0.0670179 s, 561 MB/s