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/mt7623n_bpir2_defconfig b/configs/mt7623n_bpir2_defconfig
index 4320fe5..4e8905b 100644
--- a/configs/mt7623n_bpir2_defconfig
+++ b/configs/mt7623n_bpir2_defconfig
@@ -21,6 +21,7 @@
 CONFIG_SYS_PROMPT="U-Boot> "
 CONFIG_SYS_MAXARGS=8
 CONFIG_SYS_PBSIZE=1049
+CONFIG_SYS_BOOTM_LEN=0x4000000
 CONFIG_CMD_BOOTMENU=y
 # CONFIG_CMD_ELF is not set
 # CONFIG_CMD_XIMG is not set