build_firm.sh スクリプトにより、ファームウェアファイルを作ることができます。
スクリプト本体は、ダウンロード:MA-S1xx シリーズ にあります。

user1@mas1xx-devel:~/work$ ./build_firm.sh 
MA-S1xx series Firmware build script
usage: build_firm.sh -k kernel_dir -r rootfs_dir [-c COMPRESSION] [-o firmware_file]
user1@mas1xx-devel:~/work$ 

引数として、下記を指定します。

• -k : カーネルを展開したディレクトリ
• -r : root filesystem を展開したディレクトリ
• -o : 出力ファームウェアファイル名¹⁾

実行例は下記のようになります。

user1@mas1xx-devel:~/work$ sudo ./build_firm.sh -k ~/src/linux-kernel -r mas1xx_focal_rootfs -o mas1xx_focal_v5_0_0beta1.img
MA-S1xx series Firmware build script
* use default config
  HOSTCC  scripts/basic/fixdep
  HOSTCC  scripts/kconfig/conf.o
  HOSTCC  scripts/kconfig/confdata.o
  HOSTCC  scripts/kconfig/expr.o
  LEX     scripts/kconfig/lexer.lex.c
  YACC    scripts/kconfig/parser.tab.[ch]
  HOSTCC  scripts/kconfig/lexer.lex.o
  HOSTCC  scripts/kconfig/parser.tab.o
  HOSTCC  scripts/kconfig/preprocess.o
  HOSTCC  scripts/kconfig/symbol.o
  HOSTLD  scripts/kconfig/conf
#
# configuration written to .config
#
* kernelrelease: 5.4.41-linux4sam-2020.04+
scripts/kconfig/conf  --syncconfig Kconfig
  SYSHDR  arch/arm/include/generated/uapi/asm/unistd-common.h
  SYSHDR  arch/arm/include/generated/uapi/asm/unistd-oabi.h
  SYSHDR  arch/arm/include/generated/uapi/asm/unistd-eabi.h
  HOSTCC  scripts/dtc/dtc.o
  HOSTCC  scripts/dtc/flattree.o
  HOSTCC  scripts/dtc/fstree.o
  HOSTCC  scripts/dtc/data.o
  HOSTCC  scripts/dtc/livetree.o
  HOSTCC  scripts/dtc/treesource.o
  HOSTCC  scripts/dtc/srcpos.o
  HOSTCC  scripts/dtc/checks.o
  HOSTCC  scripts/dtc/util.o
  LEX     scripts/dtc/dtc-lexer.lex.c
  YACC    scripts/dtc/dtc-parser.tab.[ch]
  HOSTCC  scripts/dtc/dtc-lexer.lex.o
  HOSTCC  scripts/dtc/dtc-parser.tab.o
... 中略 ...
	mail (8)
	staff (50)
	unknown (112)
	adm (4)
* create initramfs ...
update-initramfs: Generating /boot/initrd.img-5.4.41-linux4sam-2020.04+
  adding: kernel.img (stored 0%)
  adding: kmod.img (stored 0%)
  adding: initramfs.img (stored 0%)
  adding: rootfs.img (stored 0%)
  adding: .config (deflated 77%)
  adding: at91-centurysys-gemini-00.dtb (deflated 76%)
  adding: at91-centurysys-gemini-01.dtb (deflated 76%)
  adding: at91-centurysys-gemini-01.dtbo (deflated 64%)
  adding: at91-centurysys-gemini-02.dtbo (deflated 63%)
* Firmware filename: /home/user1/work/mas1xx_focal_v5_0_0beta1.img
done.
please remove working directory '/home/user1/work/build_1596021499' manually.
user1@mas1xx-devel:~/work$