[Nexthop] Remove /boot partition from image

[ctikku-nexthop]

Pre-submission checklist

• I've ran the linters locally and fixed lint errors related to the files I modified in this PR. You can install the linters by running pip install -r requirements-dev.txt && pre-commit install
• pre-commit run

Summary

This change removes the /boot partition from the FBOSS image. Since we are supporting btrfs for
rootfs this means the necessary modules (and their dependencies) need to be available early on
in the /boot/efi partition and creating an updated grub.cfg that loads btrfs.mod before searching
for root filesystem.

Additionally added a --debug flag to kiwi-ng build script to view debug messages during
image creation process.

Test Plan

Verified that the system does not create /boot partition: 
[root@fboss ~]# df -h
Filesystem      Size  Used Avail Use% Mounted on
devtmpfs        4.0M     0  4.0M   0% /dev
tmpfs            32G     0   32G   0% /dev/shm
tmpfs            13G   17M   13G   1% /run
efivarfs        128K   49K   75K  40% /sys/firmware/efi/efivars
/dev/nvme0n1p3  477G  967M  476G   1% /
/dev/nvme0n1p2   20M   12M  8.6M  58% /boot/efi
tmpfs           6.3G     0  6.3G   0% /run/user/0
[root@fboss ~]#

All necessary modules are copied over in /boot/efi partition:

[root@fboss ~]# ls -l /boot/efi/EFI/centos/x86_64-efi/
total 376
-rwxr-xr-x. 1 root root  64032 Jan  5 21:07 btrfs.mod
-rwxr-xr-x. 1 root root  17704 Jan  5 21:07 crypto.mod
-rwxr-xr-x. 1 root root  22872 Jan  5 21:07 diskfilter.mod
-rwxr-xr-x. 1 root root  11288 Jan  5 21:07 extcmd.mod
-rwxr-xr-x. 1 root root  13880 Jan  5 21:07 gcry_crc.mod
-rwxr-xr-x. 1 root root  18472 Jan  5 21:07 gzio.mod
-rwxr-xr-x. 1 root root  11280 Jan  5 21:07 lzopio.mod
-rwxr-xr-x. 1 root root   4392 Jan  5 21:07 raid6rec.mod
-rwxr-xr-x. 1 root root 209128 Jan  5 21:07 zstd.mod
[root@fboss ~]#