How to boot Android x86 iso with grub ( usb multiboot )?

From scratch:

1. Install grub2 on a usb
2. Extract Android x86 Iso content somewhere on usb
3. create the following grub.cfg entry

I suppose you have extracted the Android files in USB_ROOT/boot/iso/android

menuentry "Android-x86 Live" {
    set root=(hd0,msdos1)
    linux /boot/iso/android/kernel root=/dev/ram0 androidboot.hardware=android_x86 video=-16 quiet SRC= DATA=
    initrd /boot/iso/android/initrd.img
}

The above entry is created on the base of isolinux.cfg content that you find in the android iso ( you can look into this file to create installation entry as well )

Other Grub2 entry

Here some other grub2 entry that maybe helpfull in case somebody want to create a multi boot:

set timeout=10
set default=0

menuentry "Hiren 15.2" {
    linux16 /boot/grub/grub.exe --config-file="find --set-root /boot/iso/hiren_15.2/menu.lst; configfile /boot/iso/hiren_15.2/menu.lst"
} 

menuentry "Android-x86 Live" {
    set root=(hd0,msdos1)
    linux /boot/iso/android/kernel root=/dev/ram0 androidboot.hardware=android_x86 video=-16 quiet SRC= DATA=
    initrd /boot/iso/android/initrd.img
}

menuentry 'Ubuntu 14.04' {
    set isofile='/boot/iso/trusty-desktop-amd64.iso'
    loopback loop $isofile
    linux (loop)/casper/vmlinuz.efi boot=casper iso-scan/filename=$isofile locale=en_US.UTF-8
    initrd (loop)/casper/initrd.lz
}

menuentry 'Slax-7.0-kde4' {
    set dir=/boot/iso/slax
    set root=(hd0,msdos1)
    linux $dir/boot/vmlinuz from=$dir vga=normal load_ramdisk=1 prompt_ramdisk=0 printk.time=0 slax.flags=perch,xmode
    initrd $dir/boot/initrfs.img
}

menuentry "CrunchBang - Live" {
    set isofile='/boot/iso/crunchbang-11-20130506-i686.iso'
    loopback loop (hd0,1)$isofile
    linux (loop)/live/vmlinuz initrd=/live/initrd.img iso-scan/filename=$isofile toram=filesystem.squashfs findiso=$isofile boot=live config --
    initrd (loop)/live/initrd.img
}

menuentry "System Rescue CD 64" {
    set isofile='/boot/iso/systemrescuecd-x86-4.5.1.iso'
    loopback loop $isofile
    linux (loop)/isolinux/rescue64 isoloop=$isofile
    initrd (loop)/isolinux/initram.igz
}

menuentry "Clonezilla Live 2.2.3-25 amd64" --class iso {
    set isofile="/boot/iso/clonezilla-live-2.3.2-22-amd64.iso"
    loopback loop $isofile
    linux (loop)/live/vmlinuz findiso=$isofile boot=live union=aufs config noprompt ip=frommedia toram=filesystem.squashfs
    initrd (loop)/live/initrd.img
}