image.bbclass: check INITRAMFS_MAXSIZE

image.bbclass: check INITRAMFS_MAXSIZE

Usually, the initramfs' maxsize can be 1/2 of ram size since modern
kernel uses tmpfs as initramfs by dafault, and tmpfs allocates 1/2 of
ram by default at boot time, which will be used to locate the initramfs.

Set INITRAMFS_MAXSIZE to 131072K (128M) by default (ram 256M), the
initramfs is small usually, for example, core-image-minimal-initramfs is
about 21M (uncompressed, 17M * 1.3) by default, but if the user add a
lot pkgs to initramfs, we can error and stop to let the user know ealier
rather than fail to boot (e.g., OOM-killer) at boot time.

Please see the bug for more info:
https://bugzilla.yoctoproject.org/show_bug.cgi?id=5963

[YOCTO #5963]

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>