s390/decompressor: correct BOOT_HEAP_SIZE condition

Currently BOOT_HEAP_SIZE is always defined as 0x400000 due to
bogus condition. Use CONFIG_KERNEL_BZIP2 instead of
CONFIG_HAVE_KERNEL_BZIP2 to correct that.

BOOT_HEAP_SIZE of 0x10000 is still good enough for every decompressor
algorithm but bzip2. Actual decompressor memory usage with allyesconfig
is the following:
gzip  0xbc28
bzip2 0x379518
xz    0x7410
lzma  0x3e6c
lzo   0
lz4   0

Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>