Convert CONFIG_SYS_BOOTM_LEN to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_BOOTM_LEN

As part of this, rework error handling in boot/bootm.c so that we pass
the buffer size to handle_decomp_error as CONFIG_SYS_BOOTM_LEN will not
be available to host tools but we do know the size that we passed to
malloc().

Cc: Soeren Moch <smoch@web.de>
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/socfpga_stratix10_defconfig b/configs/socfpga_stratix10_defconfig
index 6aff07d..07e9f20 100644
--- a/configs/socfpga_stratix10_defconfig
+++ b/configs/socfpga_stratix10_defconfig
@@ -43,6 +43,7 @@
 CONFIG_SYS_PROMPT="SOCFPGA_STRATIX10 # "
 CONFIG_SYS_MAXARGS=64
 CONFIG_SYS_PBSIZE=2085
+CONFIG_SYS_BOOTM_LEN=0x2000000
 CONFIG_CMD_MEMTEST=y
 # CONFIG_CMD_FLASH is not set
 CONFIG_CMD_GPIO=y