Build custom openwrt-##.##.#-ramips-mt7621-mikrotik_rb750gr3-initramfs-kernel.bin

Hi experts,

I have a Miktotik RB750Gr3 router and I need to test a custom Openwrt image on it without flashing it yet.
I have used the existing openwrt-19.07.7-ramips-mt7621-mikrotik_rb750gr3-initramfs-kernel.bin in my TFTP server and it's booting fine, however I need to customize it by adding a few more packages and kernel modules.

I'm using the Image Builder and I'm able to build a custom image openwrt-19.07.7-ramips-mt7621-mikrotik_rb750gr3-squashfs-sysupgrade.bin.
However I'm not able to find any documentation about how to build the xxx-initramfs-kernel.bin image.
Please help

Afaik Image Builder cannot make initramfs images, you need to use full build system

Thanks @bobafetthotmail. I downloaded the OpenWRT SDK from here. Ran /scripts/feeds update -a followed by ./scripts/feeds install -a and then make menuconfig.
In the TUI I have marked with Y the modules and packages that I wanted to add to my custom image and saved the config as test-RB750Gr3.config
Now I'm stuck at which command I must run in order to build the initramfs image to boot my RB750Gr3 router using TFTP server. Just calling make opens the TUI again as if I haven't used it already to save the config. Maybe I'm doing something wrong but I can't identify what...

no the SDK is a tool bundle to build custom packages It is similar to full build system but it cannot build device images at all, only packages.

install prerequisite applications/tools (there are commands for most common distros)

and then install buildsystem (it's basically git clone xxxxx and then feeds update and feeds install)

Then the make menuconfig interface will show "Target Image" menu and from there you can choose to enable "ramdisk" option to build the initramfs image for your chosen device.

Then the first time you do a make it will take a while because it is first compiling the whole build chain/SDK/whatever tools it needs.
After the first time it should be faster because it is only compiling things for the device itself.

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