ArmPkg/BdsLib: Increase fallback tftp buffer size

ArmPkg/BdsLib: Increase fallback tftp buffer size

When performing a tftp download from a server which does not support
rfc2349 transfer size option (such as netkit-tftpd), the existing code
falls back to allocating an 8MB buffer. Since this is insufficient for
an uncompressed AArch64 Linux kernel image, double the size to 16MB.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ashutosh Singh <ashutosh.singh@arm.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18748 6f19259b-4bc3-4df7-8a09-765794883524